Why did Germany owe money after World War I?

Answer 1

Germany hyper-inflated their currency until it was worthless, then paid the debt in Marks with no value. This eventually caused economic crashes throughout Europe and was a major cause of the Great Depression in the US.

Answer 2

In the Treaty of Versailles Germany was compelled to accept sole legal responsibility for the out break of World War 1. Some German politicians wanted to refuse to sign the treaty and carry on fighting, but the army said that was impossible. A few politicians, like Gustav Noske even resigned. Others were haunted by that spectre of Bolshevism ...