Why was the battle of liege fought?

This oughtn't to be in US Civil War; it's a World War I question.

German military doctrine of the first World War called for beating France as quickly as possible so the entire army could be used against Russia. Liege was a city in Belgium which was basically in the Germans' way on the quickest road to France, and the Belgians didn't like the German Army using their country as a highway.