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 shot and killed four unarmed students on May 4. Nixon's responses to protesters included an impromptu, early morning meeting with them at the Lincoln Memorial on May 9, 1970. Documents uncovered from the Soviet archives after 1991 reveal that the North Vietnamese attempt to overrun Cambodia in March 1970 was launched at the explicit request of the Khmer Rouge and negotiated by Pol Pot's then-second-in-command, Nuon Chea. Nixon's campaign promise to curb the war, contrasted with the escalated bombing, led to claims that Nixon had a "credibility gap" on the issue. It is estimated that between 50,000 and 150,000 people were killed during the bombing of Cambodia between 1970 and 1973.