 Complete the following phrase:  In July 1969, Nixon visited South Vietnam, where he met with his U.S. military commanders and President Nguyễn Văn Thiệu. Amid protests at home demanding an immediate pullout, he implemented a strategy of replacing American troops with Vietnamese troops, known as Vietnamization. He soon instituted phased U.S. troop withdrawals, but also authorized incursions into Laos, in part to interrupt the Ho Chi Minh trail, which passed through Laos and Cambodia and was used to supply North Vietnamese forces. Nixon announced the ground invasion of Cambodia to the American public on April 30, 1970. Further protests erupted against what was perceived as an expansion of the conflict, and the unrest escalated to violence when Ohio National Guardsmen 