Secretary of Defense Robert McNamara announces that 21,000 more U.S. troops are to be sent to Vietnam. He also claimed that it was now known that North Vietnamese regular troops had begun to infiltrate South Vietnam. The new U.S. troops were to join the U.S. Marines and paratroopers from the 173rd Airborne Brigade that had arrived earlier to secure U.S. airbases and facilities. These forces would soon transition from defensive missions to direct combat operations. As the war escalated, more and more U.S. combat troops were sent to South Vietnam. By 1969, there were over 540,000 American troops in Vietnam.