This call for a united front, which had originated in Moscow, failed to move Chiang, but one of his most important field commanders, Chang Hsueh-liang, was not so adamant and Mao decided to work through him. Chang was known as “the Young Marshal,” since his father was Old Marshal Chang Tso-lin, whose assassination had led to the Japanese occupation of Manchuria. Though the Young Marshal commanded the Northeastern Army, which had been ordered by the Kuomintang to wipe out all Red forces in North China, he had serious reservations about Chiang’s course; he had come to believe that those he was fighting were also patriots and perhaps both sides should unite against the Japanese.

We see here how the Japanese hostilities in Manchuria and China weakened non-Communist forces.

In the fall of 1936 Mao sent his most able negotiator, Chou En-lai, to work out a truce with the Young Marshal. Chou was mild-mannered, soft-spoken, almost effeminate-looking, but it was he who had directed the gory massacres of anti-Communists in Shanghai in 1927. Like all good diplomats, he was blessed with endless patience. “No matter how angry I get,” said an old school friend named Han, “he always smiles and goes back over the same ground covered in our argument, only in a different way—different enough to make you feel as though he were presenting a new point.”

He met with Chang in a Catholic mission in Sian, a remote city in North China, and after admitting that Chiang Kai-shek was the logical leader against the Japanese, promised that the Red generals would serve under him. In return Chang would have to assure him that the Red troops get equal treatment with the Nationalists. In addition, Communists held in Nationalist prisons would be released, and the Communist party allowed to operate legally once Japan was defeated. They signed a document listing these conditions and shook hands to seal the bargain. “Young Marshal, now that it is all settled,” said Chou, “I am ready to take orders from you this very moment.”

Chang replied coldly that they would both have to wait and take orders from Chiang Kai-shek.

“If you still have any doubt about the determination of my party to join in a united front against Japan,” said Chou, “I will gladly stay here in Sian with you as a hostage.”

Chang said this wouldn’t be necessary and that he was as determined as anyone to fight the Japanese—after all, he had a personal account to settle with them. Nevertheless, he was a soldier and must first attempt to persuade his superior, the Generalissimo, to accept the terms of the truce just signed.

But before such a meeting could take place, another of Chiang’s field commanders, General Yang Hu-cheng, an ex-bandit chief, convinced the Young Marshal that the Generalissimo could only be made to co-operate with the Reds if he were kidnapped.

Oh, goody.

Chiang was already on his way to Sian to confront Chang with evidence that the Young Marshal was being influenced by leftists and to warn him that “unless timely measures were taken, the situation could lead to rebellion.”

Although he had agreed to the kidnapping, the presence of Chiang Kai-shek in Sian weakened Chang’s resolve; he continued to vacillate until General Yang took matters in his own hands on the morning of December 12. He seized the Generalissimo and all troops in the area loyal to him. Chiang had been badly injured in a fall while trying to escape, but he was more composed than the Young Marshal when they came face to face. “Both for your own sake and for the sake of the nation, the only thing for you to do is to repent at once and send me back to Nanking,” he said. “You must not fall into the trap set by the Communists. Repent before it is too late.”

It took the sheepish Chang two days to get up his nerve to show his superior a proposed eight-point agreement similar to the one made with Chou. Once it was signed, Chang promised, the Generalissimo would be escorted back to the Nationalist capital.

“So long as I am a captive, there can be no discussion,” said Chiang. He dared the other to shoot him and went back to the Bible.

The distressed Chang turned to the Reds for help. When Chou arrived he praised Chang for his courage, scolded him for bungling the kidnapping and went in to see the prisoner. They knew each other well. Chou had once served under the Generalissimo at the Whampoa Military Academy, China’s West Point; here, with Chiang’s approval, he had set up a political-commissar system. What Chiang didn’t realize until too late was that most of the commissars selected were Communists.

Chiang had since offered $80,000 for Chou’s head and was understandably pale and apprehensive. But Chou was all affability. He swore that the Communists would not exploit the situation if Chiang joined them. All they wanted was an end to civil war, and a joint effort against the Japanese.

— John Toland, The Rising Sun, © 1970, 1998.

And that's why Imperial Japan sucked. They could have gotten away with establishing a mandate in Manchuria: the late dynasty, the Qing, were Manchu, and the Chinese had no love for them, but as you can see above, 300 years of Manchu rule had inserted a lot of Manchu into China's command structure.

Had Japan offered to use a light hand in Manchuria, and support Chiang against the CHICOMs and the Soviets, history would have been very different.