WW1 Germany would probably have had about enough resources to actually conquer something like Alsace-Lorraine (which they had to begin with, I seem to recall) and similar border areas not entirely adverse to such a change of regime. As the fate of the colonial empires illustrates conquering countries for good has gotten a wee bit tricky (okay, nigh impossible) after the introduction of nationalism in its modern abstract form.

The Great War was not waged as one where the participants tried to actually conquer one another. That wasn't how people thought of Great Power wars at the time. They were out to sort out their pecking order, whose contested claim to somethingorother was stronger under ultima ratio regnum, colonial interests and suchlike. This would have been - and indeed was - done by trouncing the other guy's army (no, they really didn't quite comprehend it was going to be an industrial war; they were still thinking in essentially Napoleonic terms) so soundly he had to sue for peace and you could dictate the terms.