The election of 1800 was decided in the House of Representatives because it resulted in an electoral tie between Thomas Jefferson and Aaron Burr. Both candidates received 73 electoral votes, which led to a situation where no candidate had a clear majority needed to win the presidency outright.
According to the original provisions of the Electoral College established by the U.S. Constitution, if no candidate receives a majority of electoral votes, the election is decided by the House of Representatives. Each state delegation in the House casts one vote for one of the tied candidates, and a majority of states is required to elect the president.
The situation forced the House to conduct multiple ballots, as various factions within the House deliberated. After a contentious process, Thomas Jefferson was finally elected as the third president of the United States on the 36th ballot, largely due to the influence of several Federalists who chose not to oppose him. The election of 1800 is often referred to as a "revolution" because it marked the first peaceful transition of power between political parties in the U.S., from the Federalists to the Democratic-Republicans.