A newly released memo by Richard Nixon shows the former U.S. president worried the 1989 Tiananmen Square massacre would permanently damage relations between China and the United States.

The California-based Richard Nixon Foundation released the confidential document that Nixon sent to members of Congress after a visit to China just five months after Chinese troops crushed peaceful demonstrations in central Beijing with the loss of hundreds if not thousands of lives.

Nixon said he had spent 20 hours talking to Chairman Deng Xiaoping and other top Chinese leaders, and had found relations between the countries in the worst condition since his breakthrough visit to Beijing 17 years earlier. He said the two sides viewed the events in Tiananmen Square from “totally different perspectives” and that the gap between them was “totally unbridgeable.”

Nixon said the United States should not “excuse what happened,” but argued that Washington should not let the incident “permanently damage” its relationship with Beijing. He noted the two nations' disagreements had been even greater before his 1972 visit to Beijing, but that despite irreconcilable differences, both had recognized an overriding common interest in confronting the Soviet Union.

Nixon, who died in 1994, said “cutting ourselves off from Chinese leaders” would be detrimental to both U.S. and Chinese interests. He said the U.S. should not run the risk of becoming an “adversary” rather than an “ally” of China in the 21st century.

The release of the memo comes 40 years after President Nixon's historic visit to China, which marked the beginning of the expansion of Sino-American relations.

U.S. Secretary of State Hillary Clinton is attending a conference Wednesday in Washington to commemorate Nixon's 1972 trip. Chinese Foreign Minister Yang Jiechi will also deliver live video remarks to the conference, which is hosted by the U.S. Institute of Peace.

The U.S. and China had little contact during the 25 years before Nixon's visit. But the common threat posed by the then Soviet Union and other mutual interests helped the countries find common ground and develop a far-reaching, yet challenging, relationship.

U.S.-China relations are generally considered to have been developing positively in recent years. But the relationship between the world's two biggest economies has been dominated by mutual military suspicion, human rights concerns, trade disputes and other issues.