August 7, 1964: Congress passes the Gulf of Tonkin Resolution.
Passed in response to the Gulf of Tonkin incident, which occurred several days prior, this resolution gave President Johnson the power to “take all necessary measures to repeal any armed attack against the forces of the United States and to prevent any further aggression” without a formal declaration of war by Congress. The resolution was vital in justifying and maintaining American military involvement in Southeast Asia through the decade, even as support from all sides waned.
Eventually, in 1971, the resolution was repealed, and the War Powers Resolution was passed two years later. This new resolution was a direct reaction to the Tonkin Resolution - intended to check the President’s power by requiring him to notify and seek authorization from Congress in order to commit U.S. armed forces to any long-term conflict.
In the end, the Tonkin Gulf Resolution passed unanimously in the House of Representatives; in the Senate, only two members opposed it. One remarked:
I believe this resolution to be a historic mistake.