The Kingdom of Romania entered the war in August 1916. The Entente promised the region of Transylvania (which was part of Austria-Hungary) in return for Romanian support. The Romanian Army invaded Transylvania and had initial successes, but was forced to stop and was pushed back by the Germans and Austro-Hungarians when Bulgaria attacked them in the south. Meanwhile, a revolution occurred in Russia in February 1917 (one of the several causes being the hardships of the war). Tsar Nicholas II was forced to abdicate and a Russian Provisional Government was founded, with Georgy Lvov as its first leader, who was eventually replaced by Alexander Kerensky.