Soviet Union successfully conquers Poland in the Polish-Soviet war of 1919-1921, allowing them to assist socialist revolutionaries in Germany. This could have meant supporting the largely leftist Wiemar Republic, or a socialist insurrection against it, either way producing a Socialist, Russian-backed Germany from which to support other leftist movements in Western Europe.
This prevents "the war" in the west. The war in the East, between Japan and the other states around it may have been inevitable without a massive change of heart from the Japanese. Their new found sense of Nationalism and place in the world as a rising power drove them to emulate the worst attributes of the old European Empires. Even if the Treaty of Versailles hadn't snubbed them by limiting their fleet size, this expansionist nation-state behavior would have led them into conflict with someone eventually, even if the US had never been involved. That said, given my first paragraph, the war could have been won against Japan even without the US given the Sino-Soviet pact having the support of a heavily industrialized state like Germany, if not the rest of Europe.
Stopping Japan from invading other countries would require then to have basically gained a powerful socialist movement to make them focus on themselves instead of outward, something they almost entirely lacked in the first half of the 20th century.
Captcha: sinister Language