20 May, 1941. As the German paratroopers were jumping over Crete in order to crush the last Greek resistance against the Axis Powers, Stalin says "Go!" to Operation Thunderstrom: three million Red Army effectives, including infantry, motorized and pilots, crossed the Soviet borders in Poland and toward Romania, breaking the German defenses and advancing toward Könisberg, Warsaw, Odessa and the major Romanian oil fields.
Stalin had been preparing this offensive since the defeat in the Winter War, ordering the Army to modernize and to prepare a good offensive plan. Once Hitler had broken the ice by invading Poland, the door was open for exporting communism by force in Europe. Stalin succeeded in eastern Poland and the Baltic republics, but failed miserably in Finland. Now, the Red Army was ready. Hitler was distracted in Northern Africa and the Mediterranean, plus defending themselves from constant bombings on the homeland by Britain. According to Stalin's readings, Hitler would surely come the next year to attack the Soviet Union, so this was also a preventive strike, before German power grew too much.
What Stalin did not know was that Germany was already preparing their own invasion of Russia starting in one month, therefore having an unusual concentration of troops near the border. This was actually good, as once the surprised defenses were surpassed, the Red Army could move more easily, and as they were preparing an attack, they did not have a good defense strategy.
But despite any superiority Stalin saw in papers of the Red Army over the Wehrmacht, Stalin did not yet have the right strategy for a mobile war. By the end of June, Bacau and Könisberg had fallen; Bucharest, Warsaw and Danzig were besieged, and the German armies were regrouping.
The first of July, 1941, Germany managed to encircle the Soviet advance toward Danzig, while advancing toward Könisberg to free the city from the Soviet occupation.
On July 8, Warsaw fall to the Red Army, but faced regrouped German troops opposing any westward movement. Bucharest surrendered to the Soviets in July 10, proclaiming a new pro-Soviet government.
The encircled Soviet troops in West Prussia surrendered to the Germans in July 12. The battle for Könisberg started on July 13. The SS Waffen and the Wehrmarcht managed to encircle the Soviet occupation troops in Warsaw by July 15, breaking supply lines.
In the following months, the Soviets secured Romania. In Poland, several battles were fought but no decisive victory was achieved. Warsaw changed hands several times and by December, the Warsaw streets were the battleground.
In Prussia, however, the Germans managed to drive the Soviets away and had advanced over the Baltic republics to free them.
Despite the mistrust of Stalin, the United Kingdom was pleased that Germany had a new front that would keep them busy. Churchill was not kin to plan any offensive with Stalin, but did offer anyhow some lend leases. For the moment they were co-belligerents, rather than allies.
Intelligence showed that Operation Sealion had been officially suspended. Actually, the Blitz had been over for a while, and intelligence showed that troops had been transferring eastward even before the Russians attacked. It seems that another Battle of Britain was nothing to worry about for a while. Still, Britain kept the constant bombardment of German cities and facilities.
The strategy suggested, however, to take the heat out of the Isles, and to continue the campaigns in North Africa, the Mediterranean, and the North Sea, as well as keeping an eye on the Japanese. Enforcing the Middle East was the priority, as it was evident after the fall of Greece. Particularly as the Soviets were moving into Romania and threatening Bulgaria, Yugoslavia, and Hungary, Britain should keep the support on the Greek resistance as an step to knock Italy out of the war.
The Finish military was eager to counterattack on the Soviet Union to claim the territories lost in the Winter War. The Red Army was being defeated in the Baltic, but had managed to occupy Romania, and the Polish front was still undefined. Britain was sending aids to the Soviet Union via Murmansk. As eager as they were, no continuation war would be declared... for now.
Meanwhile in the Pacific
Japan campaigns in mainland Asia demanded oil, and Hitler had promised to supply Japan if they declared war to the Soviet Union. But Hitler did not have much oil to offer. America had been the main oil supplier to Japan, but President Roosevelt is demanding Japan to retire from China. The Dutch East Indies had oil, but is protected by both the Royal Navy and the US Navy. The Royal Navy should not be much of a problem. They are pretty much distracted in Europe., but the US Pacific Fleet was an obstacle for Japan to dominate the Pacific.
On December 8, 1941 (December 7, Hawaii time), the Japanese launch an attack on Pearl Harbor, dealing some serious damage on the US Pacific Fleet and blocking the port. The USA declares war to Japan.
Hitler takes note. Japan was a fellow Axis power, so he should honor the pact. Additionally, the USA was very friendly to Britain, providing material support, and still protecting British convoys in the Atlantic. Declaring war on the USA would justify sinking US convoys to Britain. But, on the other hand, in such a messy front with the Soviet Union, declaring war on another power, albeit one separated by an ocean, does not look clever.
Hitler demanded Yamamoto that Germany would declare war to the USA if Japan promised to declare war to the Soviet Union. The Japanese answer was elusive as usual. On December 19, Hitler was planning to announce the declaration of War on the USA, when the news came: the Red Army is invading Hungary.