Thomas Jefferson is inaugurated as the third president of the United States, becoming the first president inaugurated in Washington, D.C. Aaron Burr, who had tied Jefferson in electoral votes before losing the election in the House of Representatives, is inaugurated Vice President.

Yusuf Karamini, pasha of Tripoli, declares war on the United States by symbolically cutting down the flagpole at the U.S. consulate. This action came after the United States refused to pay more tribute to the Tripolitans in exchange for protection from piracy against American ships.

William C.C. Claiborne is appointed the new territorial governor of Mississippi.

President Jefferson delivers his first address to the newly convened seventh Congress of the United States in writing and is read aloud by the House clerk. Expressing his dislike for ceremony, Jefferson establishes the precedent, not broken until the presidency of Woodrow Wilson, of not delivering the State of the Union address in person.

The United States and Britain conclude a convention regarding Jay's Treaty of 1794 to resolve some outstanding issues about details in the treaty. A commission rules that the United States owes £600,000 to British citizens in settlement of Revolutionary War claims.

Congress recognizes the War with Tripoli, authorizing the arming of merchant ships to ward off attacks.

Congress reduces the size of the U.S. army to its 1796 limits. It also passes an act, which is signed into law by Jefferson, establishing an official United States Military Academy at West Point.

Infamous excise taxes on commodities such as whiskey are repealed.

The notorious naturalization laws of 1798 are repealed. The required length of residency reverts from fourteen years to five years.

The Georgia legislature cedes to the United States its western territory, notorious for the Yazoo land fraud of 1795.

President Jefferson signs the Enabling Act, establishing procedures under which territories organized under the Ordinance of 1787 can become a state. The law effectively authorizes people of the Ohio territory to hold a convention and frame a constitution.

Congress officially incorporates Washington as a city, empowering Jefferson to appoint the mayor.

The United States and Spain resolve to refer all disputes between the two countries to a special convention at Madrid.

Jefferson appoints James Monroe minister to France and Spain, instructing him to purchase New Orleans and East and West Florida. Napoleon informs U.S. minister in Paris Robert Livingston that France will be willing to sell the entire Louisiana territory, much to his surprise.

Ohio officially becomes the seventeenth state of the Union. It is the first state to prohibit slavery by law at its inception.

On February 24, 1803, the Supreme Court of the United States issued its unanimous ruling in Marbury v. Madison, one of the most important Supreme Court cases in early American history. This ruling established for the first time the principle that the Supreme Court can declare an act of Congress void if it is inconsistent with the Constitution. This landmark case established the basis for judicial review of congressional and executive actions on the grounds of their constitutionality.

Thomas Jefferson's election as President in 1800 came after a bitter partisan struggle between Federalists and Republicans. Republicans won both the presidency and a majority in Congress. Before leaving early in 1801, the Federalist Congress passed a new Judiciary Act that created new judgeships, which enabled outgoing President John Adams to appoint numerous additional Federalists to the judiciary. On his last day in office, Adams worked late into the night signing commissions for new judgeships.

When President Jefferson took over in March 1801, he ordered Secretary of State James Madison not to deliver the commissions. William Marbury, an appointee as a justice of the peace in Washington, sued in the Supreme Court for a writ of mandamus, or a formal order of delivery, that would compel Madison to deliver his commission. Under the 1789 Judiciary Act that instructed the Court to issue writs to government officials in such cases, Chief Justice John Marshall issued a preliminary writ. Madison ignored the writ as judicial interference with the executive branch. Marshall, an arch-Federalist, was eager to oppose Jefferson's administration but knew he could not force its submission, and wanted to assert the power of the judicial branch.

In 1803, Judge Marshall issued a clever ruling, noting that Marbury had a right to his commission, but explaining that the Supreme Court had no jurisdiction in the matter. Because the Constitution did not explicitly grant the Court power to issue writs to government officials, the Judiciary Act of 1789 was unconstitutional. Recognizing the importance of separation of powers, however, he allowed that certain political actions of the executive fell beyond court jurisdiction.

In Marbury v. Madison, the Supreme Court for the first time declared an act of Congress unconstitutional; it would not do so again until the infamous Dred Scott case in 1857. Technically, Marshall let Jefferson win the battle and protected some executive action from judicial review. His ruling announced in ringing terms, however, that the Supreme Court would assume the role of guardian of the Constitution and the nation's laws, providing a forceful check on Congress.

Spain reopens New Orleans to American merchants.

Livingston and Monroe are sent to conclude a treaty for the acquisition of New Orleans, but instead conclude a treaty for the purchase of the entire Louisiana Territory. This day marks the official signing of a peace treaty with France and the purchase of Louisiana. The addition of 828,000 square miles of land between the Mississippi and the Rocky Mountains is purchased from France for approximately $15 million, increasing the national territory by 140 percent.

Louisiana Purchase Treaty Signed

On April 30, 1803, representatives from the United States and France signed the Louisiana Purchase Treaty. The terms of the agreement gave all of the Louisiana territory from the Mississippi River to the Rocky Mountains to the United States. The Louisiana Purchase doubled the size of the United States and is considered one of President Thomas Jefferson's greatest presidential accomplishments.

In 1800, President Jefferson learned that Spain had secretly ceded Louisiana to France, and he was concerned about France attempting to reclaim its North American empire. Jefferson wanted to insure that American farmers in the Ohio River Valley had access to the Gulf of Mexico via the Mississippi River–the river was a key to the farmers' economic well-being.

In the spring of 1803, the President sent James Monroe to France to join the French minister, Robert Livingston. He instructed Monroe and Livingston to negotiate the purchase of the city of New Orleans and all or part of Florida from France for $10 million. Monroe arrived just as Napoleon I of France faced renewed war with Britain. In need of money and eager to rid himself of the hassles of governing distant lands after the successful revolt of Saint-Domingue (present-day Haiti), Napoleon offered the entire Louisiana territory to the astonished ministers. Exceeding their instructions, Monroe and Livingston agreed to pay $15 million for the whole territory. The price of $15 million amounted to approximately four cents per acre for 828,000 square miles.

Delighted with the deal but troubled because the Constitution did not specifically provide for the acquisition of new territory, Jefferson considered a constitutional amendment to allow the purchase. Napoleon threatened to withdraw the deal if not soon ratified, however, and so Jefferson sent the treaty to the Senate for approval. Federalists were furious at Jefferson's seeming hypocrisy, as he had long criticized them for not strictly interpreting the Constitution.

The constitutional dilemma for the President was substantial. While believing that the United States must expand to fulfill its republican destiny, he was the first to assert the Constitution did not authorize acquiring new territory. As would be the case in numerous issues during his presidency, Jefferson was forced seek a balance between sometimes conflicting principles. In this case, President Jefferson chose expediency and national interest when he submitted the Louisiana Purchase Treaty to the Senate. The Senate ratified the treaty in October 1803.

Jefferson commissions Commodore Edward Preble as commander of a U.S. Navy squadron sent to battle Tripoli.

Captain Meriwether Lewis, formerly Jefferson's personal secretary, sets out from Pittsburgh to begin an expedition of the newly acquired western territory of the Louisiana Purchase. Lewis will pick up Captain William Clark to serve as co-leader of the trip early in the next year. Jefferson sponsored the journey out of personal scientific curiosity and concern for the economic and political security of the western United States.

Motivated by the infamous election of 1800, Congress passes the Twelfth Amendment to the Constitution, requiring electors to vote for President and vice president separately. This ends the tradition of the runner up in a presidential race becoming vice president and prevents chances for a deadlock tie.

The French flag is lowered in New Orleans and the U.S. flag raised, symbolizing the transfer of the Louisiana territory from France to the United States.

Lt. Stephen Decatur burns the captured U.S. frigate Philadelphia while docked in Tripoli harbor. Tripolitan gunboats had captured the frigate during the previous October. No one is killed.

Congress passes the Louisiana Territory Act, dividing the Louisiana Purchase into the Territory of Orleans in the south and the district of Louisiana in the north.

On July 11, 1804, Aaron Burr and Alexander Hamilton met for duel, and Burr shot and fatally wounded Hamilton, who died the next day. The Burr-Hamilton duel stands as a vivid example that in the early republic partisan politics were also highly personal politics.

Hamilton and Burr had been political adversaries long before their famous duel. Hamilton, an arch-Federalist and President George Washington's secretary of the Treasury strongly distrusted his fellow New Yorker, and he worked against Federalist efforts to elect Burr over Thomas Jefferson, both Republicans, when they tied in the presidential election of 1800. Burr had served various positions in New York politics and then became Jefferson's vice president in 1801. Republicans felt he was unreliable, however, and dropped him in the next election.

In 1804, Burr was running for governor of New York, and Hamilton was leading the opposition to Burr's candidacy; he spoke out against him and questioned his integrity in public. For these slights, Burr challenged Hamilton to a duel. Perceiving that not responding to the challenge would destroy his own honor and render him useless in future political service, Hamilton answered Burr. Although in this era duels were usually avoided by a series of negotiations through which both parties could restore their reputations, Burr took further offense at Hamilton's response. After about ten days of correspondence, Burr and Hamilton met on July 11, 1804, in New Jersey (New York had outlawed dueling).

Hamilton had declared the previous evening his intention to fire into the air; whether or not he shot at Burr remained a point of contention for years to follow, but Burr nonetheless escaped unscathed after fatally wounding Hamilton, who died the next day. After New York and New Jersey both issued warrants for his arrest, Burr went back to the District of Columbia and resumed his position as vice president, presiding over the Senate.

As public outrage grew, Burr fled to the west, where from 1805 to 1807 he participated in a vague but ambitious plan to separate the southwest from the United States. The Supreme Court found him not guilty of treason in 1807, and after five years in Europe, Burr returned to New York, where he practiced law in New York until his death in 1836.

The Twelfth Amendment to the U.S. Constitution is officially ratified, allowing for the presidential election of 1804 to be conducted under new rules.

The fifth presidential election is held under the auspices of the newly ratified Twelfth Amendment.

Thomas Jefferson is officially reelected President of the United States. Fellow Democratic-Republican and first governor of New York George Clinton will be the vice president.

President Jefferson is inaugurated for his second term. George Clinton officially succeeds Aaron Burr as vice president. In his inaugural address, Jefferson proposes that Federalist-inspired internal taxes be completely eliminated.

Lewis and Clark reach the mouth of the Yellowstone River.

U.S. Marines and Arab mercenaries capture the Tripolitan port city of Derna, achieving a major victory for the United States in the Tripolitan War. Eaton's ultimate plan, approved by President Jefferson, entailed replacing the ruling pasha of Tripoli with the rightful ruler. This is aborted with the forthcoming peace treaty in June.

The United States and Tripoli sign a Treaty of Peace and Amity in Tripoli, effectively ending the Tripolitan War.

Rumors circulate about the subversive activities of Aaron Burr as he arrives in New Orleans. These include plans to establish a separate country with New Orleans as its capital and a plot to invade Mexico.

The British justify seizure of American ships in neutral ports with the invocation of the Rule of 1756.

Lewis and Clark reach the Pacific after a perilous journey of nearly eighteen months and 4,000 miles.

Jefferson makes two addresses, one public and one before Congress, regarding land in Florida. In the public address, Jefferson cites the need to prepare for war with Spain. Privately, Jefferson informs Congress of secret negotiations with France in order to buy the territory from them and asks for five million dollars to be appropriated. The request receives a controversial response from Congress.

Michigan is formed from the territory of Indiana.

Congress authorizes a commission to build a national road from Cumberland, Maryland, to the Ohio River.

In protest against the seizure of American ships and the impressment of American sailors by Britain, Congress passes a law prohibiting the importation of many British products into the United States.

Zebulon Pike begins his exploration of what is now the southwestern United States. On November 15, Pike explores the famous 18,000-foot peak that still retains his name in what is now Colorado.

American envoys James Monroe and William Pinckney commence talks with British official Lord Holland on the current naval hostilities.

On September 23, 1806, Meriwether Lewis and William Clark arrived back in St. Louis two-and-a-half years after they began their expedition to explore the Louisiana Territory and the Pacific Northwest. President Thomas Jefferson commissioned the expedition, which is often considered one of the greatest exploratory quests in U.S. history.

Well before President Jefferson purchased the Louisiana Territory from France, Americans had been curious about the lands west of the Mississippi River. Jefferson saw the West as a great collection of scientific specimens and a vast expanse that enhanced American security, but he also shared the commercial interest of American traders looking for a viable route to the Pacific Ocean. Even before negotiations to purchase New Orleans had commenced, Jefferson planned an expedition to the Pacific Northwest.

In January 1803, the President asked Congress for a secret appropriation of $2,500 for the secretly planned exploration. Despite Spain’s displeasure at U.S. hints that it would trek into French and Spanish territory, Jefferson appointed Meriwether Lewis and William Clark to lead the expedition. Lewis, the President’s personal secretary, was merely 28 years old but had extensive knowledge of the West. He was commissioned an Army officer and given joint command with William Clark, also an Army officer. Jefferson instructed Lewis and Clark to find a path to the Pacific Ocean, preferably via water, learn the geography of the territory, explore trade with Native Americans, and return with samples of unknown species of flora and fauna. His instructions, drafted in June 1803 before the purchase of Louisiana but with knowledge the transaction would likely occur, were implemented the following year.

Lewis and Clark left St. Louis on the Missouri River in May 1804 with a company of nearly fifty men. The Corps of Discovery voyaged up the Missouri to North Dakota, where they spent the winter before pushing on over the Rocky Mountains to the Columbia River and Pacific Ocean. When they returned to St. Louis in September 1806, they brought samples of plant and animal species Jefferson had requested and reports that Jefferson's purchase had been well worth the price.

Congress passes legislation providing for a military organizational structure.

In Washington, D.C., President Jefferson publicly warns citizens not to take part in a plot to invade Spanish territory. Jefferson issues this warning after having been told of Aaron Burr's subversive activities with respect to annexing Spanish territory.

Jefferson appeals to Congress asking for a ban on the slave trade.

Aaron Burr is arrested near Fort Stoddart, Alabama, in connection with his alleged conspiracy against the government.

At Jefferson's behest, Congress passes a law prohibiting the importation of slaves into any place within the jurisdiction of the United States after January 1, 1808.

The Embargo Act, modified and authorized by President Jefferson, now permits vessels to transport American goods from foreign ports.

The infamous Leopard incident occurs. The British ship Leopard fires upon the United States frigate Chesapeake in Chesapeake Bay after the latter's commander, James Barron, refuses to surrender four British deserters on board. Many on the U.S. frigate are killed and wounded.

Circuit court in Richmond acquits Aaron Burr of treason.

In spite of Thomas Jefferson's vehement protest, the British government announces it will continue to impress seamen on American ships thought to be British.

Britain issues its “Order in Council,” forbidding neutral nations and her allies from trading with France except under tribute to England.

Napoleon issues the Milan Decree, forbidding trade with England or her colonies under penalty of confiscation and impressments of any vessel paying tribute to Britain.

President Jefferson signs the Embargo Act, putting a halt to all trading with any country in the entire world. The act serves as a retaliatory measure to the increasingly coercive trade policies of the British and the French.

The law officially banning the slave trade goes into effect.

The Second Embargo Act comes into force. It is more stringent than the first and is commonly known as the “O grab me Act.”

Napoleon issues the Bayonne Decree, authorizing the French seizure of all U.S. vessels entering French and Italian ports and all ports of the Hanseatic League. Napoleon conveniently argues that his action helps the United States enforce its new policy prohibiting trade with other nations.

The sixth presidential election for President of the United States is held.

James Madison is elected president of the United States, with George Clinton as vice president.

After the U.S. economy suffers at the hands of the embargo, Congress repeals the Embargo Act. Jefferson signs the Non-Intercourse Act the same day, closing U.S. ports only to France and England. Trade with the two countries is to be resumed when they agreed to respect the rights of U.S. citizens and vessels.

James Madison is inaugurated as the fourth President of the United States, thereby ending Jefferson's presidency. Jefferson retires to his home at Monticello outside Charlottesville, Virginia, to assume a private life.