When World War I began, Germany invaded neutral Belgium and Luxembourg as part of the Schlieffen Plan, trying to take Paris quickly. The threat to France caused Britain to enter the war; it used the 1839 agreement as justification. The Belgian army is remembered for their stubborn resistance during the early days of the war, with the army – around a tenth the size of the Germany Army – holding up the German offensive for nearly a month, giving the French and British forces time to prepare for the Marne counteroffensive later in the year. The German invaders treated any resistance—such as sabotaging rail lines—as illegal and subversive, and shot the offenders and burned buildings in retaliation.