The correct answer is The Gulf of Tonkin Resolution.
After the Gulf of Tonkin incident during the Vietnam War, Congress agreed to give the executive branch (aka the president) significantly more power in terms of committing troops to a war. In the Gulf of Tonkin Resolution, it was stated that the president could commit military forces to a certain area without Congressional approval. Before this time, the president would have to convince Congress to declare war on a nation before comitting troops.