After the assassination of Archduke Ferdinand and Grand Duchess Sophie, Austria secured the promise of aid from its German ally and issued a list of ten ultimatums to Serbia. On July 28, 1914, Austria declared war on Serbia for failure to meet all of the demands. Russia, determined to protect Serbia, began to mobilize its armed forces. On August 1, 1914, Germany declared war on Russia to protect Austria after warnings directed at Tsar Nicholas II failed to stop Russian preparations for war.
In spite of the central European focus of the initial crises, the first blow was struck against neutral Belgium in northwestern Europe. Germany planned to take advantage of sluggish Russian mobilization by focusing the German army on France. German military leaders recycled tactics developed earlier and activated the Schlieffen Plan, which moved German armies rapidly by rail to march through Belgium and into France. However, this violation of Belgian neutrality also ensured that Great Britain entered the war against Germany. On August 4, 1914, Great Britain declared war on Germany for failing to respect Belgium as a neutral nation.
In 1915, the European war had developed into a series of bloody trench stalemates that continued through the following year. Offensives, largely carried out by British and French armies, achieved nothing but huge numbers of casualties. Peripheral campaigns against the Ottoman Empire in Turkey at Gallipoli, throughout the Middle East, and in various parts of Africa either were unsuccessful or had little bearing on the European contest for victory. The third year of the war, however, witnessed a coup for German military prospects: the regime of Tsar Nicholas II collapsed in Russia in March 1917. At about the same time, the Germans again pursued unrestricted submarine warfare to deprive the Allies of replenishment supplies from the United States.6
The Germans, realizing that submarine warfare could spark an American intervention, hoped the European war would be over before American soldiers could arrive in sufficient numbers to alter the balance of power. A German diplomat, Arthur Zimmermann, planned to complicate the potential American intervention. He offered support to the Mexican government via a desperate bid to regain Texas, New Mexico, and Arizona. Mexican national leaders declined the offer, but the revelation of the Zimmermann Telegram helped usher the United States into the war.