Another earlier PoD:
Lenin decides to keep fighting against the Germans. The war was nearly over anyways, and making peace made him pretty unpopular at home.
Being victorious in 1918 keeps his assailant away, one way or the other - so he lives much longer. Also, the SU now possesses some of the most advanced industrial areas of Germany and Austria-Hungary. The Ukraine and Poland never get independent, which also avoids the war with them and leaves much more of their industries intact. Rumanian oil fuels the Russian economy instead of the European economy.
Lenin being much more pragmatic than Stalin, the SU allows more and more free market - similar to China. That adds additional momentum to the Russian economy.
With the SU much stronger, a cold war starts earlier and a little bit hotter. Let's say Britain starts trouble from the South (Iran, India, Afghanistan...) and a war erupts between them. With a tank army much more formidable than in OTL, they "liberate" India and cause a lot of unrest in western countries. That unrest keeps the US out of the war, while most of Central Europe and the Balkans become an early Warsaw Pact.
With masses of Indian volunteers, the SU is able to liberate more and more countries in its reach. The US tries to apply more pressure against that, but as later in the case of Japan, they don't manage more than some embargos - which don't hit the SU too much, as it has enough ressources.
After some time, Britain has to give in. The SU uses the peace to expand communism to most of Asia, more parts of Europe and some parts of Africa. Officially, they don't have too much to do with those "revolts" - but the weapons of the insurgents look pretty Russian.
At about 1930, the US has put local insurgencies down and sees the SU as a great danger to their safety. Together with GB, Japan, France and some other "free" nations, they decide to attack the SU. While the allies are technologically more advanced, the SU has about the same industrial strength and many more recruitable people.
With a surprise landing in northern Canada, the US is caught off guard. At the beginning, they try to get the Russians out of Canada with aerial assaults, but that is not very effective against soldiers hopping from cover to cover. The attack also leads to weakening efforts of the Allies in Asia, so that Japanese and other Asian opposition is quickly routed by the Russians. The Communists now possess all of continental Asia and, by 1934, all of continental Europe. Wherever the Allies try to attack, they are successfully repelled.
After the US concentrates its efforts on getting the Russians out of Canada, the Russians are able to land in Alaska and Greenland, and win over a few Latin American states, from which the rest of the Americas is attacked.
The last major resistance breaks by 1938. After that, Australia, all of Africa, every populated Island, and so on are turned into communist states.
With the revolution having occurred all over the world, there is a lot of popular demand to fullfill the promises of communism - equal rights, small or no real government, local autonomy for every ethnicity and culture, and so on. This leads to a police state similar to OTL SU, which in turn leads to a few revolts. After one is successful (as in OTL), we have many independent states, wars, and so on. The SU stays pretty intact though, and plays world police in all the hot spots, including the remains of the US sometimes.