As it says on the tin. Stalin, fearing the German war machine and desiring a buffer state, reverses his decision in late August and decides he will support the territorial integrity of Poland.
Stalin makes this decision and informs his high command, but does not announce it internationally. Like the Molotov-Ribbentrop Pact. When Germany declares war on 1 September, 1939, the Red Army is ordered to cross the Polish border and help the defenders. Naturally, they ask the Polish government first, but they're pretty insistent that they should come. The USSR commits fully to defending Poland and eliminating the Reich at this time. Are they able to fight through the Nazi German troops and end the war early? How does the war develop from here?