After the USS Maddox was attacked while trying to intercept radio transmissions, President Johnson went to Congress asking for a resolution to allow him to begin engaging the North Vietnamese. The measure called the Gulf of Tonkin Resolution was passed with only two senators opposing. It gave the president carte blanche authority to conduct the war as he saw fit. After the war, Congress passed the War Powers Act to try to stop this kind of involvement in the future. Now presidents can use the troops for ninety days, but then must get a declaration of war from Congress.
Historically Speaking 