Sullivan died Oct 11 at an assisted-living facility in Washington, according to his daughter, Anne Sullivan.
As ambassador to Laos, William Sullivan controlled a secret US bombing campaign against North Vietnamese troops moving through Laotian territory along the Ho Chi Minh trail.
His daughter said he required that he approve all bombing runs, in an effort to limit civilian casualties and armed conflict in Laos.
Sullivan then served as ambassador to the Philippines and helped coordinate the arrival and eventual resettlement of thousands of Vietnamese refugees after the fall of Saigon in 1975.
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He was named ambassador to Iran in 1977 by President Jimmy Carter at a time that restive Iranians were growing tired of the rule of Shah Mohammed Reza Pahlavi. In 1979, Sullivan was instructed to tell the shah that the United States felt he should leave Iran. Ayatollah Khomeini, the exiled revolutionary leader, returned to Iran in early 1979, and revolutionary forces toppled the monarchy.
Sullivan left Iran soon afterward and retired from the Foreign Service later in 1979. Sullivan's experience as a hostage in Iran came nine months before the seizure of hostages at the US Embassy that lasted from November 1979 until the end of the Carter presidency on Jan. 20, 1981.