Complex sentence: After the war Austria lost its many non-German territories and was reduced to a rump called German Austria in 1918, a poll took place and 98% "Austrians" wanted to join the newly formed Germany but this was forbidden by the Treaty of Versailles.
Split into simpler sentences: After the war Austria lost its many non-German territories and was reduced to a rump called German Austria. Some in that state wanted to join the new German republic but this was forbidden by the Treaty of Versailles.