The Russian Empire (known commonly as Russia) is a large transcontinental country spanning most of northern Eurasia and, in this timeline, OTL Russia, Kazakhstan and most of Belarus. It is a member of the United Nations, sitting as a permanent member of the Security Council (prior to the veto's abolition in 1995 it held power of absolute veto), as well as the Council of Europe, the Pan-Slavic Conference and the Commonwealth of Former Russian States.
The Russian Empire was founded in 1721 by Peter the Great, with its capital at St Petersburg. After the near-assassination of Tsar Alexander II, the Empire, a backwards and barbaric country, underwent significant reforms, leading it, by the time of WWI, to be one of the world's largest empires, a Great Power, and a developing industrial nation focused on reform and industrialisation. During the First World War Russia sided with the Entente allies, launching an invasion of Germany and Austro-Hungary as a pre-emptive strike. It prevailed on the winning side in 1918 and subsequently settled down to a prosperous postwar period, aided by the granting of independence to Finland, Poland and the Baltic states, cleverly eliminating the need for reconstruciton in these areas.
However, the Depression caused much unemployment and discontent, facilitating the rise of Prime Minister (later Premier) Josef Dzugashvili (Stalin for propaganda purposes). Stalin became Tsar Peter V's popularity man, imposing the Emperor's will (tinged with his own strong opinions) through his own fearsome personality. The Stalinist cult of personality reached its peak in the mid-1930s, when the rise of the Nazis in Germany frightened many Russians, although his move to purge the Imperial Armed Forces of supposed German agents was shot down by an alliance of Tsar and Duma. After this, Peter and Stalin kept an uneasy balance of power, with the Tsar alternating between contempt for and fear of "that psychotic Georgian dwarf" as he called the Prime Minister in private, and Stalin paranoid of what he saw as "Fascist infiltration of holy Russian soil" and possible collaboration among the aristocracy.
World War Two, however, gave Stalin a huge "I told you so" moment, and by using the German annexation of all of mostly-independent ally Poland in 1939 as a club to beat the Tsarist faction of the Duma over the head with, elevated himself to rank of Generalissimo of the Russian Armies, a rank equalled only (supposedly) by the Tsar himself. Preparations began for defence against the Nazis, especially after the Fall of France, as well as plans for an invasion of Occupied Poland. On June 11, 1941, the Germans made their move. With the Russians unsure of whether they were going to attack or defend, the Germans were able to overrun much of the western lands quickly. Peter, seeing Stalin's immense popularity but also his military incompetence (the Generalissimo refusing to believe that the Empire was being invaded until communications were lost with Minsk and Zhitomir, and then having a nervous breakdown), decided to let Stalin keep his rank but not his power, by letting the Stavka (military high command) take control. As it was, a German thrust through the neutral Baltics endangered St Petersburg (yet again renamed Petrograd) by September, with the capital, Duma and Royal Family relocated to Kazan. Stalin moved to Moscow, regaining his fighting form after his policy of partisan warfare in the occupied areas took hold, a boost accompanied by a regaining of his ego.
Over the next four years, the Russians fought the Germans for their country, trading land for time and, when land ran out, bodies for time: time used to receive American aid and more importantly develop their armoured forces--by 1942 the heavy MT tanks were outclassing German armour, and the Katyusha rocket artillery helping the Empire win the grinding Battle of Tsaritsyn. Through 1943 and '44 the Russians rolled back the Axis in East Europe, relieving Rostov, Kharkov, Smolensk, Kharkov again, Kiev and then Petrograd, after 900 days of siege. By August 1944 the 1000-Year Reich was falling apart, allowing Stalin to unleash August Storm, the invasion of Japanese-held China. By the same time next year the war was won, but at a heavy price: 15 million citizens of the Empire dead, 3 million of those Jews and Russian untermenschen killed in the Nazi camps. This underscored post-war policies of repression and revenge, to pay back the Germans and their allies for all they did to Russia.