- In the 1800 election, Jefferson and Aaron Burr tied in the electoral college vote, which had to be decided by the Federalist-controlled House of Representatives, where they finally chose Jefferson.
- Jefferson purchased the Louisiana Territory from France in 1803, doubling the size of the US but raising constitutional questions about executive power.
- Jefferson imposed an embargo on foreign trade from 1807-1809 to remain neutral in the Napoleonic Wars, but it mainly hurt New England and failed to change British and French policy.