Perhaps the most unexpected achievement of Lenin and Stalin was their success in holding together the vast multiracial Russian Empire created by the Tsars, so that in the end it outlasted all the other European world-empires.
In the two centuries before the First World War the Tsars expanded their empire enormous distances from its ethnic Russian base around Moscow. It now stretched all the way across Siberia to the Pacific, and took the Amur provinces from the Chinese Empire. The Baltic provinces, Finland and most of Poland were incorporated, as were the Christian kingdoms beyond the Caucasus, and the Moslem lands of Central Asia, with their ancient cities of Samarkand and Bokhara. By 1914 ethnic Russians made up less than half the population of this vast empire. Everywhere the Tsars encouraged the local ruling elites to work within the imperial structure in return for their wealth being guaranteed: Baltic German nobles, Cossack warriors, Georgian clan chiefs, even Moslem emirs. In only a few cases did the Russians meet long-term resistance. It took half a century to compel the Chechens and other tribes of the Caucasus mountains to accept Russian rule, and the Jews, of whom great numbers lived in the western regions of the empire, were always subject to discrimination and occasional persecution, with the result that hundreds of thousands of Jews emigrated, and others became revolutionaries. The Poles never accepted Tsarist rule, and from time to time they rose in rebellion and were crushed with great ferocity. (I once visited a Polish museum in Berkshire. The theme was entirely anti-Russian: you would never have learnt from it that Germany had twice overrun Poland in the past century; nor was there any mention of the once-great Jewish community in Poland)
Lenin and the Bolsheviks seized power in 1917, but initially they controlled only the ethnic Russian heartland around Moscow and up to Petrograd. Vast areas had to be surrendered to the Germans by the Treaty of Brest-Litovsk in early 1918, but after a vicious and enormously costly civil war, and the collapse of the German Empire, it turned out that almost all the old Tsarist empire had been recovered. Finland and the Baltic lands had been lost. The trans-Caucasian territories of Georgia, Armenia and Azerbaijan attempted to break away, but wasted their energies quarrelling amongst themselves, and were forcibly incoporated into the Soviet Union a few years later. Poland, however, successfully beat off an invasion. By this time, however, the empires of Germany, Austria and Turkey had collapsed, but the Russian Empire was still there, albeit now in Communist garb. The national minorities in the frontier areas, particularly in the Ukraine, suffered particularly in Stalin's collectivisation of agriculture and Great Purge.
The end of the Second World War saw Stalin not only regain almost all the territories lost earlier, but also establish a cordon of client-states in central Europe, where any sign of independence was ruthlessly crushed. Only Finland of the old Tsarist empire now remained outside Soviet control. What was left of the ancient Russian Jewish community was once again persecuted in Stalin's last years, and several of the more awkward racial minorities; Chechens, Crimean Tartars and others; were deported to Siberia, where enormous numbers of them died before Khrushchev eventually let them return to their homelands. Local nationalism was a topic never to be officially discussed in the Soviet Union.
In the end, the Tsarist-Soviet Empire outlived even the British and French empires, and only came to an end with the collapse of communism in 1989-91. But the behaviour of Putin's government in Chechnia, Georgia and the Ukraine suggests that Russia's rulers are even now not fully reconciled to their empire's demise.