What event led to the 1964 gulf of tonkin resolution?

Alleged attacks by North Vietnam against US naval ships in the Gulf of Tonkin.

Detail:

The Gulf of Tonkin Resolution was a measure passed by US Congress that allowed the US President to make military actions, like increase troops, without formal declaration of war.  It led to huge escalation of US involvement in the Vietnam War. The resolution was passed by Congress in August, 1964, after alleged attacks on two US naval ships in the Gulf of Tonkin.  The key wording in the resolution said:

That resolution served as a blank check for President Johnson to send troops to whatever extent he deemed necessary in pursuance of the war.  Between 1964 and the end of Johnson's presidency in 1969, US troop levels in Vietnam increased from around 20,000 to over 500,000.