Although Japan joined the fight against the Central Powers in World War I, her military efforts were made against allies as well as enemies. Using Korea as a beachhead, she attempted to enlarge her empire on the Asiatic mainland at the expense of Russia, then in the throes of revolution. Three years after the Armistice, a Japanese army still occupied the Vladivostok area; but the United States took such a firm diplomatic stand that Tokyo backed down.
This retreat was only a postponement. During the next decade Japan set up a strategic shield to the east and south by fortifying the mandated islands of the Pacific, awarded to her after the war. Treaties and agreements were violated whenever convenient, and in 1931 she turned westward again to satisfy her appetite for Russian and Chinese territory.
The time was well chosen. With the Western nations in the depths of an industrial depression, Japan began a series of aggressions against the Chinese in Manchuria. The gains were consolidated in a puppet state known as Manchukuo, comprising a fertile and populous area as large as California. China was unable to offer much resistance, and Soviet Russia could not risk a major war in the Far East. Even so, some of the Soviet border clashes with the Japanese in time of “peace” were actually battles fought with tanks and planes.
In 1937 came the Japanese invasion of China proper. Germany and Italy were launching aggressions of the same stamp in Europe and Africa, and the world was to know little stability until all three totalitarian states had been crushed in World War II.
Soviet Russia had a grim struggle for survival while resisting the full tide of Nazi invasion. But at the time of the Yalta Conference, Stalin was in a position to ask a stiff price for military aid in the Pacific. The United States agreed that the Port Arthur area and southern Sakhalin should be returned to Russia to redress the “wrongs” of 1905. Concessions were also made in Manchuria and outer Mongolia.
Stalin, for his part, consented to sign a treaty of friendship with Nationalist China as an ally of the United States. Later events made it evident that he had no intention of keeping his pledges. On the contrary, Soviet policy already visioned a Communist empire in the Far East which would include China as well as Korea.