World War I
The assassination of Austrian Archduke Franz Ferdinand on June 28, 1914, set in motion a series of alliances that drew Europe into total war. Germany used a new weapon, submarine warfare, to cut supplies to the Allies. The Central Powers included Imperial Germany, Austria-Hungary, Bulgaria, and the Ottoman Empire. Opposing them were the Allied Powers of France, Great Britain, and Russia. Italy, which had been politically connected with Germany and Austria-Hungary, entered the war on the Allied side in 1915.
On May 15, 1915, a German U-boat torpedoed the Lusitania, killing over 1,000 people. Americans were outraged that Germany destroyed an unarmed passenger liner without warning—a violation of rules of engagement. To prevent US entry into the war, Germany ends unrestricted submarine warfare.
Fearing America's entry into the war, in January 1917, the German foreign secretary sent a secret telegram to the Mexican government guaranteeing German aid if Mexico agreed to go to war with the United States. Germany would assist Mexico in reclaiming its former territories of Texas, New Mexico, and Arizona. British code-breakers intercepted the note and leaked the contents to the U.S. government.
With the declaration of war, the military reforms undertaken between 1903 and 1916 were put to the test. On June 12, 1917, the National Guard was federalized. New York regiments rushed to bring their ranks to wartime strength.
When the National Guard was federalized for the war in Europe, units were renumbered to avoid overlap and confusion. Numbers 1 through 100 were reserved for Regular Army Regiments. National Guard Regiments were to be numbered from 101 to 300.