The United States of America moved unalterably toward war on March 1, 1917, when President Woodrow Wilson released the Zimmermann Telegram to the press.

When World War I broke out in Europe in August 1914, President Wilson stated America's commitment to neutrality. As Europe quickly settled into trench warfare and stalemate on the battlefield, two power blocs emerged — the Allied Powers consisting of Britain, France, Italy and Russia, and the Central Powers made up of Germany, Austria-Hungary and the Ottoman Empire.

When the European war began, public opinion in the United States did not heavily favor one side or the other. Plenty of Americans of Irish and German background applauded the Central Powers and their war against England. Indeed, in 1914 there was nothing in America's historical relationship with Britain to suggest that the two would one day become the closest of allies. Winning independence from Britain in 1783, America fought Britain again during the War of 1812, and Britain had been decidedly hostile to the Union, though neutral, during the U.S. Civil War.

Yet, as the war dragged on America did find itself drifting toward Great Britain and the Allies over the Central Powers. There are several reasons that explain this. First of all, Congress passed a series of neutrality laws which stated that any belligerent power could buy war material from the United States, provided they shipped it to Europe themselves. As Britain ruled the waves, and its dominion Canada was next door to the U.S., that meant that the Allies rather than the Central Powers benefited from these laws.

As the war progressed the Allies bought more and more from U.S. firms on credit, ensuring that if they lost the war, American businesses would never collect on their contracts. Additionally, Britain proved skilled at propaganda, and exaggerated German atrocities in order to inflame American opinion.

In his book, “The Illusion of Victory: America and World War I,” historian Thomas Fleming wrote: “A flood of stories portrayed the Germans as monsters capable of appalling sadism. Eyewitnesses described infantrymen spearing Belgian babies on their bayonets as they marched along, singing war songs. Accounts of boys with amputated hands (supposedly to prevent them from using guns) abounded. ... At British expense, a group of Belgians toured the United States telling these stories. Woodrow Wilson solemnly received them in the White House.”

These stories, however, were not true. (One of the unfortunate effects of this British propaganda was that years later, during World War II, when the very real details of the Nazi death camps came to light, many Americans dismissed them as more British fairy tales.)

The first major event that impacted American public opinion, however, was the May 1915 sinking of the British passenger liner, the Lusitania. The ship had been sunk by a German U-boat under the German policy of unrestricted submarine warfare. Approximately 1,200 people died, among them 120 Americans. In the wake of the event, and amid fears of America entering the war against them, Germans declared they would abandon the controversial policy.

Several key events occurred in February 1917 that would set America on a collision course with Germany. First of all, many Americans recoiled at the thought of allying themselves with Imperial Russia, one of Britain's partners in the war. The Russian government of Czar Nicholas II was even more autocratic and hostile to democracy than was that of Germany, the prospective enemy. This American attitude changed when the Russian Revolution broke out that February, and Russia appeared as though it would become a republic. (The Bolsheviks under Vladimir Lenin would not come to power until October of that year.) The odium of allying with an autocratic state disappeared.

Second, the German high command, desperate for any kind of advantage over the Allies, resumed unrestricted submarine warfare that month, enraging many Americans and igniting fears of another Lusitania disaster. Even after these events, however, most Americans still favored neutrality.

The event that tipped the scales arrived in the form of a diplomatic cable sent from Germany to Mexico. German foreign minister Arthur Zimmermann had convinced the Kaiser that war with America had become a real possibility, and Germany needed to take precautions.

In the awkward language of a coded telegram, Germany's note read in part: “We offer Mexico on following terms alliance. ... Generous financial support and understanding on our part that Mexico in Texas, New Mexico, Ar(izona) ... former lost territory back conquer.”

The Germans were offering the Mexican government a deal. If America declared war on Germany, Germany wanted Mexico to declare war on the United States and distract it from the war in Europe. In exchange, Germany would help Mexico recover Texas, New Mexico, and Arizona — much of the territory lost during the Mexican-American War of 1848. What the Germans didn't count on, however, was that Great Britain had tapped Germany's sole underwater cable to the New World, decrypted the telegram, and gave it to the U.S. government.

Mexican President Venustiano Carranza dismissed the idea as fanciful, knowing that Mexico was in no position to challenge American power at the time. (Mexico had been unable to stop an American expeditionary force hunting the bandit Pancho Villa within its borders a few years earlier.)

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After Wilson released the telegram to the press on March 1, many Americans initially believed the telegram was simply more British propaganda. Fleming writes: “Most diplomats ... would have dismissed the dispatch as a forgery. But the Germans, imbued with the righteousness of their cause and profoundly disenchanted with the Americans for selling their enemies hundreds of millions of dollars' worth of high explosives and weaponry, admitted every word was true.”

The note sent shock waves throughout the nation. In her book “The Zimmermann Telegram,” historian Barbara W. Tuchman wrote: “Zimmermann's admission shattered the indifference with which three-quarters of the United States had regarded the war until that moment. The nation sat up and gasped, 'They mean us!' Nothing since the outbreak of war had so openly conveyed a deliberately hostile intent toward Americans, and nothing had so startled opinion across the country.”

The Zimmermann Telegram was seen as evidence that Germany was plotting against the United States, and that its diplomatic relations since the outbreak of war had been conducted in bad faith. In response to this revelation, and the other factors stated above, Woodrow Wilson went before Congress on April 2 to ask for a declaration of war against Germany. Four days later the United States was formally at war.

Cody K. Carlson holds a master's degree in history from the University of Utah and currently teaches at Salt Lake Community College. He is also the co-developer of the popular History Challenge iPhone/iPad apps. Email: ckcarlson76@gmail.com