What was the gulf of tonkin resolution

The Gulf of Tonkin Resolution authorized the American president to "take all necessary measures to repel any armed attack against the forces of the United States."
This gave full power to the president to deflect any attacks against the U.S. However, many people believed that this gave the president more power than he should have. Due to this, the War Powers Act was later passed. It required the president to consult with congress within 48 hours of sending troops into a conflict.
This ensured that congress would agree with the president's decision and that it would not put the United States in any additional danger. 

Was a resolution that the United States Congress passed on August 7, 1964, in response to the Gulf of Tonkin incident.

From mexican american war , us gained new mexico.