President Richard Nixon warns South Vietnamese President Nguyen Van Thieu in a private letter that his refusal to sign any negotiated peace agreement would render it impossible for the United States to continue assistance to South Vietnam. Nixon’s National Security Advisor Henry Kissinger had been working behind the scenes in secret negotiations with North Vietnamese representatives in Paris to reach a settlement to end the war. However, Thieu stubbornly refused to even discuss any peace proposal that recognized the Viet Cong as a viable participant in the post-war political solution in South Vietnam. As it turned out, the secret negotiations were not close to reaching an agreement because the North Vietnamese launched a massive invasion of South Vietnam in March 1972. With the help of U.S. airpower and advisers on the ground, the South Vietnamese withstood the North Vietnamese attack, and by December, Kissinger and North Vietnamese representatives were back in Paris and close to an agreement. Among Thieu’s demands was the request that all North Vietnamese troops had to be withdrawn from South Vietnam before he would agree to any peace settlement. The North Vietnamese walked out of the negotiations in protest. In response, President Nixon initiated Operation Linebacker II, a massive bombing campaign against Hanoi, to force the North Vietnamese back to the negotiating table. After 11 days of intense bombing, Hanoi agreed to return to the talks in Paris. When Kissinger and Le Duc Tho, the main North Vietnamese negotiator, met again in early January, they quickly worked out a settlement. The Paris Peace Accords were signed on January 23 and a cease-fire went into effect five days later. Again, President Thieu refused to sign the Accords, but Nixon promised to come to the aid of South Vietnam if the communists violated the terms of the peace treaty, and Thieu agreed to sign. Unfortunately for Thieu and the South Vietnamese, Nixon was forced from office by the Watergate scandal in August 1974, and no U.S. aid came when the North Vietnamese launched a general offensive in March 1975. South Vietnam succumbed in 55 days.