In World War I, Germany declared war on France as part of a long-held strategy called the Schlieffen Plan. The Schlieffen Plan required German troops to organize on the frontier of Belgium, a neutral country, with the idea to invade France.
The day before officially declaring war, the German government gave Belgium and its king, King Albert, an ultimatum to give passage to the German army or to be invaded. Germany officially declared war on France on August 3, 1914. The conflict that originally led to Germany's Schlieffen Plan was the assassination of Archduke Ferdinand and his wife, Susan, by Serbian nationalists in the Balkans region.