Isolationism is part of the founding creed of U.S. foreign policy, as articulated by those with the most vested in its conduct. Two oft-cited quotes illuminate the prevailing thinking at the birth of a nation newly formed out of the colonial aspirations of a distant empire:
"It is our policy to steer clear of permanent alliances with any portion of the foreign world";
"Peace, commerce, and honest friendship with all nations -- entangling alliances with none"
The first quote, by George Washington in his "Farewell Address," and the second, by Thomas Jefferson in his inaugural address, have been broadly interpeted as providing the moral and intellectual foundation for an isolationist foreign policy. While the basic theme proved consistent into the 20th Century, U.S. foreign policy was repeatedly compelled to interact with the world out of its own self-interest.
Distaste for permanent alliances was easy to use as a tenet of U.S. foreign policy for over a century. As the United States became more involved in European politics, however, isolationism became too slim a reed on which to base a coherent policy. Whether the United States should have become militarily involved in the first world war is worthy of debate. As the 19th Century drew to a close, American presidents began to see U.S. interests increasingly tied to foreign developments. The United States' industrial capacity laid the groundwork for a stronger country and increasingly assertive foreign policy. The growth of the Navy allowed for a more active role abroad, as manifested in the aforementioned Spanish-American War, which saw U.S. naval forces battle the Spanish for control of the Philippines and Cuba.
Woodrow Wilson's election to the presidency marked a transformation in U.S. foreign policy, a major shift towards formal alliances. When war broke out in Europe in 1914, engulfing the continent in horrific trench warfare with the introduction of bombing from the air, the use of automatic weapons, and, of particular concern, the use of chemical weapons, Wilson felt increasingly compelled to act. While much of the country continued to oppose U.S. involvement in European conflicts, German submarine activities in the Atlantic, culminating in the sinking of the British passenger ship Lusitania, killing over one hundred Americans, and growing sympathy for the British and French presented Wilson the freedom to ally the U.S. with those two countries.
Isolationism returned in force between the two world wars. The American public, and its representatives in Congress, remained isolationist. Four times during the 1930s, Congress passed legislation, known as the Neutrality Acts of 1935, 1936, 1937, and 1939, reaffirming the sentiment that Europe's wars should be left to Europe. That sentiment was shattered irreversably with Japan's attack on U.S. forces at Pearl Harbor, Hawaii, and in the Philippines. Until then, President Franklin Roosevelt had been at a loss to convince the public of the necessity of confronting growing German power, and he was limited to supporting the British, and Russians through maritime shipments of military and civilian aid. The devastation and costs of World War II convinced most American policy-makers that the United States could no longer afford to resist alliances, and the North Atlantic Treaty Organization (NATO) has been at the center of U.S. foreign policy ever since.