February 13, 1965, Johnson authorized Rolling Thunder, the sustained bombing of North Vietnam. On March 8, 1965, two Marine battalions, 3,500 troops, went ashore near Da Nang to protect the airfields, with orders to shoot only if shot at—this was the first time U.S. combat forces had been sent to mainland Asia since the Korean War. On April 3, Johnson authorized two additional Marine battalions, one Marine air squadron, and an increase in logistical support units of 20,000 men. He also authorized troops to go on active "search and destroy" missions. By mid-April, Marines had moved to full-scale offensive operations. By November 1965, there were 175,000 troops and by 1966, an additional 100,000. The number would surge to 535,000 by the end of Johnson's presidency.