 Complete the following phrase:  Though Serbia was defeated in 1915, and Romania joined the Allied Powers in 1916, only to be defeated in 1917, none of the great powers were knocked out of the war until 1918. The 1917 February Revolution in Russia replaced the Monarchy with the Provisional Government, but continuing discontent with the cost of the war led to the October Revolution, the creation of the Soviet Socialist Republic, and the signing of the Treaty of Brest-Litovsk by the new government in March 1918, ending Russias involvement in the war. Germany now controlled much of eastern Europe and transferred large numbers of combat troops to the Western Front. Using new tactics, the German March 1918 Offensive was initially successful. The Allies fell back and held. The last of the German reserves were exhausted as 10,000 fresh American troops arrived every day. The Allies drove the Germans back in their Hundred Days Offensive, a continual series of attacks to which the Germans had no countermove. One by one, the Central Powers quit: first Bulgaria September 29, then the Ottoman Empire October 31 and the Austro-Hungarian Empire November 3. With its allies defeated, revolution at home, and the military no longer willing to fight, Kaiser Wilhelm abdicated on 9November and Germany signed an 