The event that in the spring of 1970 stirred a new round of antiwar protests was the

The correct answer is Invasion of Cambodia

Although the war had been localized to Vietnam, in 1970 the Southern Vietnamese army with the help of the United States troops had invaded into Cambodia who was also having the same problems as the Vietnamese, the communists were fighting those against communism, so Vietnamese people decided to join the fight, which stirred a new round of protests.