A naval attack that probably never happened helped start the Vietnam War

In August 1964 the US reported North Vietnamese boats had attacked American destroyers in the Gulf of Tonkin on two separate nights. Congress used the second, disputed attack to pass a resolution granting the president sweeping war powers in Vietnam. A declassified 2005 NSA history concluded that intelligence was misread and the second attack almost certainly never occurred.

— Wikipedia contributors, Gulf of Tonkin incident — Wikipedia entry on the Gulf of Tonkin incident, citing NSA historian Robert J. Hanyok's declassified 2005 analysis