The United States first imposed sanctions on Iran in 1979 after Iranian revolutionaries took American hostages at the U.S. embassy in Tehran. These initial sanctions froze Iranian assets and included a ban on weapons sales. Sanctions were lifted in 1981 as part of a deal for the hostages' release. Additional sanctions were imposed throughout the 1980s and 2000s in response to Iran's support of terrorism and refusal to halt uranium enrichment. In 2015, the United States and other world powers reached a deal lifting sanctions in exchange for limits on Iran's nuclear program, but in 2018 President Trump withdrew the U.S. from the agreement and reimposed sanctions.