A War on Two Fronts

Germany and the Central Powers were divided between 2 major fronts - the Western and the Eastern.  In what ways were theses fronts different?

The Inevitable Start to WWI

The international tensions present at the start of the 20th century included:  Nationalism, Alliance System, Economic Rivalry and Arms Race.  Which do you believe was the most instrumental cause of the First World War?  Give a brief justification for your opinion.