Well during its earlier years the USSR did try to annex many of its neighbours. Even while the Russian civil war was raging the USSR tried to reclaim Poland in 1919 but where beaten back by 1921. Though this annexation failed the Soviets still carried on trying to annex former Russian Imperial territories and they went on to annex the short lived Ukrainian National Republic and the Belarusian people's republic. In fact, many of the Russian Empire's former territories declared independence following its collapse and the soviets attempted to annex them back into the fold albeit under soviet control. However, they didn't reclaim all the lost territory and provinces such as Bessarabia were absorbed into Romania while the Baltic states remained independent and so did Finland. However, come the Second World War the Soviets annexed the Baltic states in 1940 as well as retaking the provinces that had become part of Romania. As well as that the soviets invaded Finland, a former part of the Russian Empire, and after a Pyrrhic victory they annexed Karelia which to this day remains part of Russia proper.

Also, the Soviets did annex the Japanese part of Sakhalin and the Kuril Islands north of Japan, although they didn't make a move on the Japanese home islands themselves.

That's all I know of but there may be more, they did briefly occupy Manchuria although this was returned to Communist China and before the GDR was established East Germany was technically run directly by the Soviets though that wasn't permanent. They did however annex and keep a chunk of East Prussia which today is the Russian exclave of Kaliningrad.

After WW2 Soviet Union tried to annex part of Iran and retreated only after Iraninan Shah asked UN for help.