February 15 - Harmony Society is officially formed.
March 1 - Justice Samuel Chase is acquitted at the end of his impeachment trial by the U.S. Senate.
March 17 - The Italian Republic, with Napoleon as president, becomes the Kingdom of Italy, with Napoleon as King.
April 7 - Lewis and Clark Expedition: The Corps of Discovery breaks camp among the Mandan tribe and resumes its journey West along the Missouri River.
April 7 - First public performance of Beethoven's Third Symphony (Eroica).
April 26 - United States Marines captured Derne, Tripoli under the command of First Lieutenant Presley O'Bannon.
April 27 - First Barbary War: United States Marines and Berbers attack the Tripolitan city of Derna (The "shores of Tripoli" part of the Marines' hymn).
May 23 - Napoleon Bonaparte is crowned King of Italy with the Iron Crown of Lombardy in the Cathedral of Milan.
May 26 - Napoléon Bonaparte assumes the title of King of Italy and is crowned with the Iron Crown of Lombardy in the Duomo di Milano gothic cathedral in Milan.
June 10 - First Barbary War: Yussif Karamanli signs a treaty ending hostilities with the United States.
June 11 - A fire consumes large portions of Detroit in the Michigan Territory.
June 13 - Lewis and Clark Expedition: scouting ahead of the expedition, Meriwether Lewis and four companions sight the Great Falls of the Missouri River.
June 30 - The U.S. Congress organizes the Michigan Territory.
July 22 - Napoleonic Wars: War of the Third Coalition
October 14 - Battle of Elchingen, France defeats Austria
October 21 - Napoleonic Wars: Battle of Trafalgar: A British fleet led by Admiral Lord Nelson defeats a combined French and Spanish fleet off the coast of Spain under Admiral Villeneuve. It signalled the virtual end of French maritime power and left Britain navally unchallenged until the twentieth century.
October 21 - Napoleonic Wars: Austrian General Mack surrenders his army to the Grand Army of Napoleon at Ulm, reaping Napoleon over 30,000 prisoners and inflicting 10,000 casualties on the losers. Ulm was considered to be one of Napoleon's finest hours.
November 1 - Napoleon Bonaparte invades Austria during the War of the Third Coalition.
November 11 - Napoleonic Wars: Battle of Dürenstein
November 16 - Napoleonic Wars: Battle of Schöngrabern
November 26 - Official opening of Thomas Telford's Pontcysyllte Aqueduct.
December 2 - Napoleonic Wars: Battle of Austerlitz
December 3 - Lewis and Clark Expedition mark their explorations from the Missouri River overland to the Columbia River on a pine tree.
December 26 - Austria and France sign the Treaty of Pressburg.
