 By March 1965, American combat troops began arriving in South Vietnam, first to protect the airbases around Chu Lai and Da Nang, later to take on most of the fight as "[m]ore and more American troops were put in to replace Saigon troops who could not, or would not, get involved in the fighting".As fighting escalated, widespread aerial and artillery bombardment all over North Vietnam by the United States Air Force and Navy began with Operation Rolling Thunder.On 8-9 April 1965, Ho made a secret visit to Beijing to meet Mao Zedong.