On November 11, 1918, the armistice between Germany and the Western powers was signed in a railway carriage in Compiègne, ending the First World War. Ironically, Hitler would use the same cart years later to accept the French surrender - he was a funny guy like that. Anyhow, this is the first of Germany's issues - crumbling allies.
loads of reasons for the German surrender - many of the reasons affected both sides, here's a few (excuse the cartoon, I was going to put it up to be annoying but it's very informative)
The combined effect of these factors was the German defeat - which in turn gave birth to a tremendous deceit which would prove to be a major catalyst for World War II