September 1st, 1939. France crosses the border into Alsace-Lorraine, beginning World War II. Soviet forces are caught by surprise by this reckless act, but seeing decisive strategic superiority on the COMINTERN's side, Leon Trotsky declares war on Germany the next day. The Red Army throws itself at the Ostwall, a network of fortifications spread across Eastern Europe. On the 5th of September the Soviet 1st Guards' Tank Army, commanded by General Georgy Zhukov, breaks through Eisenpakt defences in Eastern Prussia, following the Baltic coastline into the heart of Germany. Behind them, numerous cavalry and motorised divisions perform the first large-scale deep battle operations, wreaking havoc on western supply routes. Incessant airstrikes prevent Eisenpakt forces from regrouping or resupplying efficiently.
However, they are not unopposed. Panzergruppe "Kleist", led by Marshal Ewald von Kleist sallies out from Königsberg to meet them with 10 panzerdivisione at its disposal. Harsh fighting ensues, with Kleist's unit receiving 30-60% casualties including two divisions being wholly annihilated. The 1st Guards' Tank Army sees half of its units destroyed, but still outnumbers the Germans and is able to continue limited offensive operations while Panzergruppe "Kleist" is redeployed to Germany's industrial heartland so it may be replenished and refit.
With the Deutsches Heer's armoured forces out of the equation, Soviet forces continue pushing nearly unopposed. The 1st Guards' Tank Army is instrumental in attacking across the Northern European Plain and forces German units to the south to retreat, lest they be surrounded and destroyed wholesale. A notable exception is the 14. Armee, which continued fighting alongside Polish and Yurizlansian forces in the First Battle of Galicia.