by capturing Tchepone, and it was spun off as an "operational success". Easter Offensive and Paris Peace Accords, 1972 Vietnamization was again tested by the Easter Offensive of 1972, a massive conventional PAVN invasion of South Vietnam. The PAVN quickly overran the northern provinces and in coordination with other forces attacked from Cambodia, threatening to cut the country in half. U.S. troop withdrawals continued, but American airpower responded, beginning Operation Linebacker, and the offensive was halted. The war was central to the 1972 U.S. presidential election as Nixon's opponent, George McGovern, campaigned on immediate withdrawal. Nixon's National Security Advisor, Henry Kissinger, had continued secret negotiations with North Vietnam's Lê Đức Thọ and in October 1972 reached an agreement. President Thieu demanded changes to the peace accord upon its discovery, and when North Vietnam went public with the agreement's details, the Nixon administration claimed they were attempting to embarrass the president. The negotiations became deadlocked when Hanoi demanded new changes. To show his support for South Vietnam and force Hanoi back to the negotiating table, Nixon ordered Operation Linebacker II, a massive bombing of Hanoi and Haiphong 18–29 December 1972. Nixon pressured Thieu to accept the terms of the agreement or else face retaliatory military action from the U.S. On 15 January 1973, all U.S. combat activities were suspended. Lê Đức Thọ and Henry Kissinger, along with the PRG Foreign Minister Nguyễn Thị Bình and a reluctant President Thiệu, signed the Paris Peace Accords on 27 January 1973. This officially ended direct U.S. involvement in the Vietnam War, created a ceasefire between North Vietnam/PRG and South Vietnam, guaranteed the territorial integrity of Vietnam under the Geneva Conference of 1954, called for elections or a political settlement between the PRG and South Vietnam, allowed 200,000 communist troops to remain in the south, and agreed to a POW exchange. There was a sixty-day period for the total withdrawal of U.S. forces. "This article", noted Peter Church, "proved... to be the only one of the Paris Agreements which was fully carried out." All U.S. forces personnel were completely withdrawn by March 1973. U.S. exit and final campaigns, 1973–1975 In the lead-up to the ceasefire on 28 January, both sides attempted to maximize the land and population under their control in a campaign known as the War of the flags. Fighting continued after the ceasefire, this time without US participation, and continued throughout the year.