Already and well before the outbreak of WWI, Russia had become one of the largest land empires on Earth, exercising hegemony over people of many nationalities, religions and ethnicities. The Russian Imperial territories had been extended southwest to the Austrian-Hungarian Empire; east to the Pacific; south and east absorbing the Caucasus Mountains and Central Asia; and north all the way to the polar icecap.
Massive public suffering and discontent during WWI led to the resignation of the imperial monarchy of Russia. The Russian Provisional government and its Soviet successors made feeble attempts to continue the War and end it on terms favorable to Russia. The result was significant territorial losses to her western and southern boundaries. The Russian people and newly formed Communist/Bolshevik government viewed these losses as a humiliation.
WWI witnessed the end of several established empires including not only Tsarist Russia, but also the German and the Ottoman Empires. Russia, now the Soviet Socialist Republic, endured a three year Civil War. After the Bolshevik victory and the collapse of the Ottomans, Lenin successfully reclaimed many of the losses in Central Asia. These territories became Soviet Socialist Republics and joined together under a new name: the Union of Soviet Socialist Republics (USSR).
In the interwar period Lenin and Stalin regained most of the territories lost during and after WWI. This process was marked by the invasion in Central Asia and Asia minor, the Molotov-Ribbentrop Pact of 1939 dividing Poland, and by the export of the Russian revolution to these areas.
During and after WWII, the Soviet Union continued to gain control of territories through invasion and reparations. The Yalta Conference (1943) allowed Stalin enough opportunities to gain control of several more central European countries. Fearing a revitalized and vengeful Germany as well as an ideologically hostile West, Stalin looked to use these countries as a defensive buffer and as a springboard for exporting the Revolution further west. Less then a century after disruptions of WWI and the Russian Revolution, the new Russian / Soviet Empire peaked with 15 republics, six Warsaw Pact states, and might again be described (by western historians) as the largest ‘empire’ on earth.
The glory of Soviet Russia was short lived. The cost of the Empire was unsustainable: the parallel arms race with the US and China and the stagnant Soviet economy led to an implosion of the Soviet fiscal and political system. The collapse of the Soviet State in the early 1990s led to the creation of some nation states that had previously been ethnic regions. Other territories that had been states before, such as Poland, where given new borders. However, many of the territories had never been independent, either dominated by Russia or else another empire. In total 15 republics and six Warsaw Pact countries became independent in 1991.