Which event resulted in President Johnson sending additional troops to Vietnam without a formal declaration of war?

Gulf of Tonkin incident

Operation Rolling Thunder

election of 1964

Viet Cong invasion in 1965

The answer would be : Gulf of Tonkin Incident

The incident became really complicated because both side of Vietnam and United states claimed that they were being attacked first. According to US' report, the ships from the northern Vietnam were the first one who opened the fire. Using this report as a basis, in the end President Johnson action was justificated