A collection of documents relating to the "Xi'an Incident" in 1930s China is coming to auction at Bonhams.

The 1936 coup saw the nationalist Chinese general Zhang Xueliang force his military leader, Chiang Kai-shek, into house arrest in the city of Xi'an.

This imposition compelled Chiang Kai-shek to negotiate with the communist leader, Mao Zedong, in the hope of bringing an end to hostilities between the two sides and enabling them to focus their attention on the invading Japanese army.

A peace accord, featuring Mao Zedong's signature but not Chiang Kai-shek's

Following two weeks of negotiations, an informal agreement was achieved (but never signed by Chiang Kai-shek), allowing the two sides to prevail against Japan and Chiang Kai-shek to leave his home.

Zhang Xueliang immediately feared for his safety, and employed former Hollywood stunt man Hyland "Bud" Lyon to act as his bodyguard.

It is from Lyon's personal collection that these documents, which have never before been shown in public, now emerge.

They include an April 1936 three page letter to Zhang Xueliang from Mao Zedong suggesting that the two sides should work together. It has a $500,000 estimate ahead of the San Francisco auction on December 11.

Two copies of the peace accord between the nationalists and the communists also feature. Both signed by Mao Zedong but not by Chiang Kai-shek, they have a combined $700,000 valuation.

Zhang Xueliang received 10 years' imprisonment from the military court for his part in the coup, although he spent the majority of his life under house arrest until his death in 2001.

Lyon left for the US in 1941.

Chairman Mao memorabilia is a constant presence at auction. A first edition, first print of Quotations from Chairman Mao Zedong, better known as the Little Red Book, sold for £6,600 ($10,500) in London in June 2006.

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