"State Sponsors of Terrorism" is a designation applied by the United States Department of State to countries which the Department alleges to have "repeatedly provided support for acts of international terrorism." Inclusion on the list imposes strict unilateral sanctions.
The countries currently on the list are Iran, Sudan, Syria, and North Korea.
The list began on December 29, 1979, with Libya, Iraq, South Yemen, and Syria. Cuba was added to the list on March 1, 1982, and Iran on January 19, 1984. Later North Korea in 1988 and Sudan on August 12, 1993, were added. South Yemen was removed from the list in 1990, Iraq was removed twice in 1982 and 2004, Libya was removed in 2006, North Korea was removed in 2008 and added again in 2017, and Cuba was removed in 2015.
Iran was added to the list on January 19, 1984. According to Country Reports on Terrorism 2013:
Iran views Syria as a crucial causeway in its weapons supply route to Hizballah, its primary beneficiary. In 2013, Iran continued to provide arms, financing, training, and the facilitation of Iraqi Shia fighters to the Assad regime’s brutal crackdown, a crackdown that has resulted in the death of more than 100,000 civilians in Syria. Iran has publicly admitted sending members of the IRGC to Syria in an advisory role. There are reports indicating some of these troops are IRGC-QF members and that they have taken part in direct combat operations. In February, senior IRGC-QF commander Brigadier General Hassan Shateri was killed in or near Zabadani, Syria. This was the first publicly announced death of a senior Iranian military official in Syria. In November, IRGC-QF commander Mohammad Jamalizadeh Paghaleh was also killed in Aleppo, Syria. Subsequent Iranian media reports stated that Paghaleh was volunteering in Syria to defend the Sayyida Zainab mosque, which is located in Damascus. The location of Paghaleh’s death, over 200 miles away from the mosque he was reported to be protecting, demonstrated Iran’s intent to mask the operations of IRGC-QF forces in Syria.