400th Missile Squadron, Strategic missile unit at Francis E. Warren Air Force Base, Wyoming, United States.

The 400th Missile Squadron was a strategic missile unit of the United States Air Force stationed at Francis E. Warren Air Force Base in Wyoming. It operated intercontinental ballistic missiles as part of the nation's nuclear deterrence system.

Activated in April 1942 as the 10th Reconnaissance Squadron at Key Field, Mississippi, it was renamed the 400th Bombardment Squadron within a week and flew B-24 Liberators from Australia against New Guinea, the Bismarck Archipelago and the Philippines, earning two Distinguished Unit Citations. Inactivated in 1946, it returned in July 1964 at Francis E. Warren Air Force Base with fifty Minuteman I missiles, received Minuteman IIIs from 1973, and was inactivated in October 2005 after the Peacekeeper's retirement.

The squadron's name and lineage tie it to the 90th Bombardment Group, whose World War II Liberator crews earned two Distinguished Unit Citations and a Philippine Presidential Unit Citation. In 1999 the 400th received the General Samuel C. Phillips Award as the best missile squadron in Air Force Space Command.

The squadron was inactivated in 2005, so there is no active unit to visit at Francis E. Warren Air Force Base. The Peacekeeper missiles it operated were retired on 19 September 2005 under the Strategic Arms Reduction Treaty. The base remains an active military installation with restricted access.

The 400th was the only Air Force squadron ever to place the LGM-118A Peacekeeper on alert, from 30 December 1986 until the missile's retirement on 19 September 2005. In 1999 it received the General Samuel C. Phillips Award as the best missile squadron in Air Force Space Command. Under the Strategic Arms Reduction Treaty its Peacekeepers were cut to a single reentry vehicle in 2001.