On 6 April 1917 the United States formally joined a wartime coalition for the first time in its history. In refusing actually to join the Anglo-French-Russian alliance, President Woodrow Wilson hoped to avoid even an implied commitment to the many secret treaties that provided for the division of the spoils among the Allied Powers, although he also realized that American public opinion would support the idea of continuing some measure of aloofness from European political systems.
American entry into World War I supported Theodore Roosevelt's contention. Whether the cause was German submarine warfare, American national security, business investments in Europe, or a desire to control events, the United States obviously if unknowingly had accepted his argument that it had to "play a great part in the world." Inspired by Wilson's rhetoric about a world safe for democracy, Americans set out upon their own "Great Crusade." After the defeat of Germany and its allies, the United States hoped to reform Europe and establish a permanent peace. Frustrated by the slow pace of reform at home, many Progressive Era reformers looked to Europe and the world for new opportunities. Coalition diplomacy during the war reflected American distrust of Europe. It took the pressure of a German offensive to get U.S. generals to coordinate their actions with a newly created Allied commander in chief, and even then the United States refused to permit its troops to come under foreign command.
Woodrow Wilson's historic proposal for the League of Nations has rightfully dominated the history of the postwar period. Wilson's concept of collective security, however incompletely developed, clearly represents one of the few attempts by a major world statesman to find a workable substitute for the diplomacy of power politics—alliance, coalition, and entente. Wilson's proposal had a fatal flaw: it rested upon the creation of a homogeneous world economic-political system. The collective security approach required a remarkable degree of cooperation and trust among the major world powers, but such trust could develop only when they shared similar political and economic creeds, and that was not to be.
Instead, the peace settlements that followed World War I created a system of alliances and ententes by which the victors hoped to preserve the status quo. Although the United States refused a role in Europe when it rejected membership in the League of Nations, a proposed alliance with France against Germany might well have received Senate approval, but the Wilson administration lost interest in it following the rejection of the Treaty of Versailles. It soon became "traditional" again for Americans to speak disdainfully of Europe's power politics, never realizing that their government continued to display a strong interest in European events. In fact, American "observers" at the league's meetings frequently attempted to influence the deliberations, and throughout the 1920s and 1930s American policy paralleled that of Britain and France.
The peacekeeping system in Europe operated without overt American support, but the system for Asia sprang primarily from the efforts of the United States. The Washington Naval Conference of 1921–1922, called by Secretary of State Charles Evans Hughes, resulted in a series of treaties, each of which involved the United States in Asian power politics. The Five-Power Naval Disarmament Treaty was aimed directly at ending the naval arms race between Japan, Britain, and the United States. The Four-Power Treaty between Britain, Japan, France, and the United States replaced the old Anglo-Japanese alliance with one that promised only consultations. Both agreements clearly implied American support for the status quo in the Pacific. The Nine-Power Treaty, which merely endorsed the Open Door in China, served to distract critics from the realities of the power relationships being established. American participation in this informal system had one limitation: there could be no prior commitments (entangling alliances?) requiring the use of either economic or military coercion.
The onset of the Great Depression in 1929 eliminated whatever slim chance there might have been of that system developing into a meaningful and long-term entente. Moreover, Germany, China, and the Soviet Union, all excluded from the power structure, soon mounted challenges that spelled the demise of the informal system that had spurned them. The 1930s saw most nations withdraw into themselves, but none more so than the United States. Embittered and cynical about their experience in Europe and the international community following the Great Crusade, Americans indulged in self-recrimination and vowed never again to try to "save" Europe from itself.
Despite the rising tension caused by Nazi Germany during the early and mid-1930s, Americans opposed any participation by their government in European politics. President Franklin D. Roosevelt, although concerned about the actions of Adolf Hitler, chose to follow the lead of Britain and France. Those nations, eager to avoid a military confrontation, repeatedly asked the United States for firm commitments. The pattern held for all of Hitler's and Benito Mussolini's aggressive moves right up until war began. The remilitarization of the Rhineland in 1935, the Italian invasion of Ethiopia in 1936, the intervention in the Spanish Civil War beginning in 1936, German Anschluss with Austria in 1938, and the takeover of Czechoslovakia in 1939, all saw the United States draw away from Anglo-French requests for some sort of alliance. Inaction resulted as each blamed the other for a lack of leadership. Whether an alliance would have prevented a conflict with Germany is questionable; so is the claim that American support would have made the British and the French more courageous in their diplomacy. What is not questionable was the American attitude toward an alliance. The general public, Congress, and most public leaders believed that alliances caused wars instead of preventing them, and they opposed any such arrangements for the United States.