A group of activists calling themselves the Emergency Peace Federation visited White House on February 28, 1917, to plead with their longtime ally, President Woodrow Wilson. Think of his predecessors George Washington and John Adams, they told him. Surely Wilson could find a way to protect American shipping without joining Europe’s war.

If they had met with him four months earlier, they would have encountered a different man. He had run on peace, after all, winning re-election in November 1916 on the slogan “He kept us out of war.” Most Americans had little interest in sending soldiers into the stalemated slaughter that had ravaged the landscapes of Belgium and France since 1914. Wilson, a careful, deliberative former professor, had even tried to convince England and Germany to end World War I through diplomacy throughout 1916. On January 22, speaking before the U.S. Senate, he had proposed a negotiated settlement to the European war, a “peace without victory.”

What the peace delegation didn’t fully realize was that Wilson, caught in a series of events, was turning from a peace proponent to a wartime president. And that agonizing shift, which took place over just 70 days in 1917, would transform the United States from an isolated, neutral nation to a world power.

“The President’s mood was stern,” recalled Federation member and renowned social worker Jane Addams, “far from the scholar’s detachment.” Earlier that month, Germany had adopted unrestricted submarine warfare: Its U-boats would attack any ship approaching Britain, France, and Italy, including neutral American ships. The peace delegation hoped to bolster Wilson’s diplomatic instincts and to press him to respond without joining the war. William I. Hull, a former student of Wilson’s and a Quaker pacifist, tried to convince Wilson that he, like the presidents who came before him, could protect American shipping through negotiation.

But when Hull suggested that Wilson try to appeal directly to the German people, not their government, Wilson stopped him.

“Dr. Hull,” Wilson said, “if you knew what I know at the present moment, and what you will see reported in tomorrow morning’s newspapers, you would not ask me to attempt further peaceful dealings with the Germans.”

Then Wilson told his visitors about the Zimmermann Telegram.

“U.S. BARES WAR PLOT,” read the Chicago Tribune’s headline the next day, March 1, 1917. “GERMANY SEEKS AN ALLIANCE AGAINST US; ASKS JAPAN AND MEXICO TO JOIN HER,” announced the New York Times. German foreign minister Arthur Zimmermann’s decoded telegram, which Wilson’s administration had leaked to the Associated Press, instructed the German ambassador in Mexico to propose an alliance. If the U.S. declared war over Germany’s unrestricted submarine warfare, Zimmermann offered to “make war together” with Mexico in exchange for “generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona” (ceded under the Treaty of Guadalupe Hidalgo that ended the Mexican-American War nearly 70 years earlier).

Until the dual shocks of unrestricted submarine warfare and the Zimmermann Telegram, Wilson had truly intended to keep the United States out of World War I. But just 70 days later, on April 2, 1917, he asked Congress to declare war on Germany. Wilson’s agonized decision over that period permanently changed America’s relationship with the world: He forsook George Washington's 124-year precedent of American neutrality in European wars. His idealistic justifications for that decision helped launch a century of American military alliances and interventions around the globe.

In his January speech, Wilson had laid out the idealistic international principles that would later guide him after the war. Permanent peace, he argued, required governments built on the consent of the governed, freedom of the seas, arms control and an international League of Peace (which later became the League of Nations). He argued that both sides in the war—the Allies, including England and France, and the Central Powers, including Germany—should accept what he called a “peace without victory.” The alternative, he argued, was a temporary “peace forced upon the loser, a victor’s terms imposed upon the vanquished.” That, Wilson warned, would leave “a sting, a resentment, a bitter memory” and build the peace on “quicksand.”

But nine days later, at 4 p.m. on January 31, the German ambassador in Washington informed the U.S. State Department that his nation would begin unrestricted submarine warfare—which threatened American commerce and lives on the Atlantic Ocean—at midnight. “The President was sad and depressed,” wrote Wilson’s adviser Edward House in his diary the next day. “[He] said he felt as if the world had suddenly reversed itself; that after going from east to west, it had begun to go from west to east and that he could not get his balance.”

Wilson cut off diplomatic relations with Germany, but refused to believe war was inevitable. “We do not desire any hostile conflict with the Imperial German Government,” he told Congress on February 3. “We are the sincere friends of the German people and earnestly desire to remain at peace with the Government which speaks for them. We shall not believe that they are hostile to us unless and until we are obliged to believe it.”

Though most Americans weren’t eager to fight, Wilson’s critics raged at his inaction. “I don’t believe Wilson will go to war unless Germany literally kicks him into it,” former President Theodore Roosevelt, who had failed in his bid to re-take the White House in 1912, wrote to U.S. Senator Henry Cabot Lodge.

Then, on February 23, came the “kick.” That day, the British government delivered a copy of the Zimmermann Telegram to Walter Hines Pace, the American ambassador in London. It was the espionage coup of the war. Britain’s office of naval intelligence had intercepted and partially decoded it in January, and a British spy’s contact in a Mexican telegraph office had stolen another copy on February 10. Pace stayed up all night drafting a message to Wilson about the telegram and its origins. When Zimmermann’s message arrived from London at the State Department in D.C. on Saturday night, February 24, Acting Secretary of State Frank L. Polk took it directly to the White House. Wilson, Polk recalled later, showed “much indignation.”

Four days later, when Wilson met with the peace activists, he revealed that his thoughts about how to bring about a lasting peace had changed. He told them, according to Addams’ recollection in her memoir, that “as head of a nation participating in the war, the President of the United States would have a seat at the Peace Table, but that if he remains the representative of a neutral country he could at best only ‘call through a crack in the door.’”

The telegram inflamed American public opinion and turned the nation toward war. Yet even then, the deliberative Wilson was not quite ready. His second inaugural address, delivered March 5, asked Americans to abandon isolationism. “We are provincials no longer,” he declared. “The tragic events of the 30 months of vital turmoil through which we have just passed have made us citizens of the world. There can be no turning back. Our own fortunes as a nation are involved whether we would have it so or not.” Today, Wilson’s address reads like a prelude to war—but at the time, pacifists like Addams heard it as a continuation of his focus on diplomacy.

When Wilson met with his cabinet on March 20, he was still undecided. But two events the previous week added to his calculus. German U-boats had sunk three American ships, killing 15 people. And the ongoing turmoil in Russia had forced Nicholas II to abdicate the throne, ending 300 years of Romanov rule. The czar’s abdication had ceded power to a short-lived provisional government created by the Russian legislature. That meant that all of the Allied nations in World War I were now democracies fighting a German-led coalition of autocratic monarchies.

The cabinet unanimously recommended war. Wilson left without announcing his plans. “President was solemn, very sad!” wrote Secretary of the Navy Josephus Daniels in his diary.

Wilson likely made his decision that night. On March 21, he set a date with Congress for a special session on April 2 on “grave matters of national policy.” Alone, Wilson wrote his speech by hand and by typewriter.

According to a story that appears in many Wilson biographies, the president invited his friend Frank Cobb, editor of the New York World, to the White House on the night before his speech. Wilson revealed his anguish to his friend. He’d tried every alternative to war, he said, and he feared Americans would forsake tolerance and freedom in wartime. In words that echoed his speech to the Senate, Wilson said he still feared that a military victory would prove hollow over time.

“Germany would be beaten and so badly beaten that there would be a dictated peace, a victorious peace,” Wilson said, according to Cobb. “At the end of the war there will be no bystanders with sufficient power to influence the terms. There won’t be any peace standards left to work with.” Even then, Wilson said, “If there is any alternative, for God’s sake, let’s take it!” (Cobb’s account, given to two fellow journalists and published after his death in 1924, is so dramatic that some historians think it’s not authentic. Other historians find it credible.)

On April 2, when Wilson came to the podium at the Capitol, no one but House and perhaps Wilson’s wife, Edith, knew what he would say. He asked Congress to “declare the recent course of the Imperial German Government to be in fact nothing less than war against the government and people of the United States,” and to “formally accept the status of belligerent.” He recounted Germany’s submarine attacks and called the Zimmermann Telegram evidence of “hostile purpose.” He also declared the German government a “natural foe of liberty.” His speech’s most famous phrase would resound through the next century, through American military victories and quagmires alike: “The world must be made safe for democracy.”

Cheers resounded through the House chamber. Later that week, Congress declared war, with 373-50 votes in the House and an 82-6 margin in the Senate.

But after the speech, back at the White House, Wilson was melancholy. “My message today was a message of death for our young men,” Wilson said—and then broke into tears. “How strange it seems to applaud that.” (His secretary, Joseph Tumulty, recorded the president’s words in his 1921 memoir. But as with Cobb’s dramatic anecdote, there is doubt among historians about the story’s veracity.)

All in all, 116,516 Americans died in World War I among about nine million deaths worldwide. (More would die from the flu epidemic of 1918 and pneumonia than on the battlefield.) Wilson’s own administration struck blows against freedom and tolerance during the war, imprisoning anti-war activists such as socialist Eugene Debs. And at the Versailles conference of 1919, Wilson became one of the victors dictating peace terms to Germany. His earlier fears that such a peace would not last eerily foreshadowed the conflicts that eventually erupted into another world war.

Wilson’s high-minded argument that the U.S. should fight World War I to defend democracy has been debated ever since. A different president might have justified the war on simple grounds of self-defense, while diehard isolationists would have kept America neutral by cutting its commercial ties to Great Britain. Instead, Wilson’s sweeping doctrines promised that the United States would promote stability and freedom across the world. Those ideas have defined American diplomacy and war for the last 100 years, from World War II and NATO to Vietnam and the Middle East. A century later, we’re still living in Woodrow Wilson’s world.