Well, at the start of WW2, only Bukovina and Bessarabia were part of the Soviet Union. But later on, a lot of Northern Romania was.

Romania

Because Soviet Union invaded Bessarabia and Northern Bucowina as a result of the Ribbentrop-Molotov secret pact.

* 1916 - Romania declared war on Austria-Hungary, Germany and Bulgaria, and was defeated. * 1918 - Romania gained Transylvania and the Banat from Austria-Hungary. * 1919 - Romanian forces played a key role in putting down Communist revolution in Hungary. * 1918-1920 - Romania acquired Bessarabia from the emerging Soviet Union.

Romania joined World War 1 in 1916 on the Entente side. Though defeated, it gained Transylvania, Banat and Bukovina from Austria-Hungary and Bessarabia from Russia.