The War of 1812 was a military conflict that lasted from June 1812 to February 1815, fought between the United States of America and the United Kingdom, its North American colonies, and its Native American allies. Historians in the United States and Canada tend to see it as a war in its own right, but the British often see it as a minor theatre of the Napoleonic Wars. By the war's end in early 1815, the key issues had been resolved and peace returned with no boundary changes.

The United States declared war due to trade restrictions brought about by the British war with France, the impressment of as many as 10,000 American merchant sailors into the Royal Navy, British support for Native American tribes fighting European American settlers on the frontier, outrage over insults to national honor during the Chesapeake–Leopard Affair, and interest in the United States in expanding its borders. The British government, which felt it had done everything in its power to try to avert the war, were dismayed by the American declaration, and believed it to have been an opportunistic ploy by President James to annex Canada while Britain was fighting a war with France. The view was shared in much of New England. The primary British war goal was to defend their North American colonies.

With the majority of its land and naval forces tied down in Europe fighting the Napoleonic Wars, the British used a defensive strategy until 1814. Warships and privateers of each side attacked the other's merchant ships, and the British blockaded the American coast. Meanwhile, early victories over poorly-led U.S. armies demonstrated that the conquest of the Canadas would prove more difficult than anticipated. Despite this, the U.S. was able to inflict serious defeats on Britain's Native American allies, ending the prospect of an independent Indian confederacy in the Midwest under British sponsorship. U.S. forces took control of Lake Erie in 1813, and seized western parts of Upper Canada, but further American offensives aimed at Montreal failed, and the war also degenerated into a stalemate in Upper Canada by 1814. In April 1814, with the defeat of Napoleon, Britain now had large numbers of spare troops and adopted a more aggressive strategy, launching invasions of the United States. An invasion of New York was defeated at Plattsburgh, and a second force, although successfully capturing Washington, was ultimately repulsed during an attack on Baltimore, inspiring the United States national anthem, "The Star-Spangled Banner". Both governments were eager for a return to normality and peace negotiations began in Ghent in August 1814. These repulses led Britain to drop demands for a native buffer state and some territorial claims, and the preliminary signing of the Treaty of Ghent took place in December 1814, although news failed to arrive before the British suffered a major defeat at New Orleans in January 1815. The U.S. later ratified the treaty, which returned occupied land, prisoners of war and captured ships (with the exception of warships due to frequent re-commissioning upon capture) to their pre-war owners and resumed friendly trade relations without restriction.

In the United States, late victories over invading British armies produced a sense of euphoria over a "second war of independence" against Britain, bringing about the "Era of Good Feelings." The war was also a major turning point in the development of the U.S. military, with militia being increasingly replaced by a more professional force. The U.S. also acquired permanent ownership of Spain's Mobile District.

In Upper and Lower Canada, British and local Canadian militia victories over invading U.S. armies promoted the development of a distinct Canadian identity. Today, memory of the war retains significance, because the defeat of the invasions ensured that the Canadas would remain part of the British Empire, rather than be annexed by the United States. The conflict has not been commemorated on nearly the same level in the modern-day United States, though it is still taught as an important part of early American history, and Dolley Madison and Andrew Jackson's respective roles in the war are especially emphasized. The war is remembered in Britain primarily as a footnote in the much larger Napoleonic Wars occurring in Europe, which effectively ended in 1815 at the Battle of Waterloo.