The Gulf of Tonkin Resolution was a resolution passed by the United States which stated that the president (Lyndon B. Johnson) was authorized, without a formal declaration by Congress, to use conventional military force in Southeast Asia. In fact, it authorized the president to do "whatever necessary" during this intervention.
The Gulf of Tonkin Resolution was an attempt to circumvent the constitution. According to the Constitution, only Congress has the power to declare war. However, Johnson was worried that a declaration of war would be too strong of an attack on North Vietnam. He also believed that this would increase pressure on the President to act in a certain way. By passing this resolution he was able to engage in acts of war at his discretion and without the need for the approval of Congress.