In researching your question, it appears the Gulf of Tonkin Resolution is the act of Congress which gave President Johnson authority to place U.S. troops in a combat role in Vietnam. Remember it was a resolution of Congress, not a law. It was later repealed. However, the actual piece of legislation that gave President Johnson the authority was Article II, section II of the United States Constitution in which the president is acknowledged as "commander-in-chief". Of course, Congress controls the purse strings so they could effectively end almost any conflict by refusing to approve funding. Hence, the system of checks and balances.
