Since 1783 the United States had acquired a number of foreign policy traditions. George Washington, in his Farewell Address, admonished his young and vulnerable country to avoid alliances that would drag it into disputes in which it had no interest. Thus was born a powerful isolationist and exclusivist tradition. The Monroe Doctrine declared the Western Hemisphere off-limits to European adventurism, giving birth to a regionalist and paternalist tradition vis-à-vis Latin America. After the Civil War, belief in America’s Manifest Destiny directed national attention to the West Coast and beyond. Then the war against Spain in 1898 yielded colonial possessions in the Caribbean and Pacific and inspired the building of a two-ocean navy and of a Panama Canal to serve it. By 1914, when the canal opened, the United States was already the greatest industrial power in the world, yet its tradition of exclusivity and its tiny standing army gave the Europeans excuse to ignore America’s potential might.

In August 1914 President Woodrow Wilson implored the American people to be “neutral in thought as well as deed” with respect to the European war. In so doing he was not only honouring tradition but also applying his own religious principles to foreign policy. His agenda upon entering the White House in 1913 had been domestic reform, and he had written that it would be an irony of fate should foreign policy come to dominate in his administration. Yet when fate so decreed, Wilson preferred to trust his own motives and methods rather than the advice of his secretaries of state or his other advisers. Wilson deplored the war and earnestly wished to bring about a just and lasting peace through U.S. mediation, for what greater mission could Providence assign to that “city on a hill,” the United States of America?

American power began to figure in the balance of war almost from the start. Trading was suspended on the New York Stock Exchange when war broke out, but when it resumed in November 1914, Europeans sold most of the $4,000,000,000 worth of securities they held before the war. U.S. loans to belligerents were at first declared “inconsistent with the true spirit of neutrality,” but the large Anglo-French orders for U.S. munitions, raw materials, and food created an economic boom, and by 1915 the Allies needed credit to continue their purchases. An initial £200,000,000 loan in September 1915 led eventually to billions being floated on the U.S. market and a complete reversal of the financial relationship between the Old World and the New. By 1917 the United States was no longer a debtor nation but the world’s greatest creditor. U.S. firms also inherited many overseas markets, especially in Latin America, which the British and Germans could no longer serve.

To Americans neutrality seemed both moral and lucrative—the United States, said Wilson, was “too proud to fight.” But the failure of his peace initiatives, the German assaults on neutrals’ rights at sea, and the cumulative effect of Allied propaganda and German provocations conjoined to end U.S. neutrality by 1917. On February 4, 1915, Germany declared the waters around the British Isles a war zone in which Allied ships would be sunk, without warning if necessary. While this procedure dispensed with traditional civilities like boarding, search and seizure, and care of civilians, effective submarine warfare required it. Underwater craft relied on stealth and surprise and exposed themselves to easy destruction once they made their presence known. Thus, even though the British blockade interfered with neutral shipping more than the German blockade, the latter appeared far more beastly. The sinking of the Cunard liner Lusitania on May 7, 1915, which killed over a thousand passengers, including 128 U.S. citizens, outraged U.S. public opinion despite the rightful German claim that she was carrying munitions (173 tons worth). Two more passenger ships, the Arabic and Hesperia, went down in August and September, respectively, whereupon American diplomatic protests caused civil officials in Berlin to overrule the military command and call off unrestricted submarine warfare, although the issue did not remain settled.

Wilson’s own peace initiatives, including an offer of mediation by Secretary of State William Jennings Bryan in 1914 and a trip to Europe by Wilson’s personal aide and adviser, Colonel Edward M. House, in 1915, were unsuccessful. Early in 1916 House returned to Europe and on February 22 in London agreed to a formula whereby the United States would summon a peace conference and—if Germany refused to attend or proved unreasonable—“would leave the conference as a belligerent on the side of the Allies.” Wilson later drew back from the guarantee and added the word “probably” after “would.” But the British themselves shied from promoting such a conference, while the other belligerents also ducked the suggestion lest they compromise the determination of their people or incur the distrust of allies.

By the end of 1916 Germany had 102 U-boats ready for service, many of the latest type, and the chief of the naval staff assured the kaiser that unrestricted submarine warfare would sink 600,000 tons of Allied shipping per month and force Britain to make peace within five months. Bethmann fought to delay escalation of the submarine war in hopes of another Wilsonian peace move. But the president held off new initiatives during his reelection campaign. When he had still not acted by December 1916, Bethmann was forced to make a deal with his own military, which consented to tolerate a German peace offer in return for Bethmann’s endorsement of unrestricted submarine warfare if the offer failed. But the army helped ensure that the German note (released December 12) would fail by insisting on implicit retention by Germany of Belgium and other battlefield conquests. Wilson followed on the 18th with an invitation to the two camps to define their war aims as a prelude to negotiation. The Allies demanded evacuation of occupied lands and guarantees against Germany in the future. The Germans stuck to their December note, and the military command decided to resume unrestricted submarine warfare on February 1.

The United States broke diplomatic relations with Germany on February 3 and commenced the arming of merchant ships on March 9. Meanwhile, German Foreign Secretary Arthur Zimmermann, anticipating war with the United States over the U-boat issue, cabled an offer of alliance to Mexico on January 16, promising Mexico its own “lost provinces” of Texas, Arizona, and New Mexico in case of war with the United States. British intelligence intercepted the Zimmermann telegram and leaked it to Washington, further inflaming American opinion. When U-boats proceeded in mid-March to sink the Algonquin, City of Memphis, Vigilancia, and Illinois (the latter two without warning), Wilson went before Congress and in a lofty and moving address reviewed the reasons why America was forced to take up the sword—why, “God helping her, she can do no other.” On April 6, 1917, Congress declared war on Germany, and the United States became an associated (not an Allied) power. Henceforth World War I hinged on whether the U-boats could force Britain to her knees and the German armies overwhelm the sagging Western Front before the men and matériel of the aroused Yankees could arrive in France.