D) Franco-Prussian War was a major cause of tension between Germany and France at the beginning of World War I. The Franco-Prussian War took place in 1870-1871 and resulted in the defeat of France and the establishment of the German Empire. As part of the peace settlement, France was forced to cede the region of Alsace-Lorraine to Germany, which became a source of tension and bitterness between the two countries in the following decades. This territorial dispute was one of the main reasons for France's hostility towards Germany leading up to World War I.