The isolationist's beliefs, however, no longer reflected the realities of the world situation. The United States had acquired a far greater stake in the international power balance and exerted far more influence on it than the isolationists were prepared to admit. Neutrality legislation did not reduce this influence but simply redirected it, not necessarily into desirable channels.
In general, the American policy gave aid and comfort to would-be aggressors since it offered tacit assurance that this country would not actively oppose their actions as long as they did not directly threaten the United States. More specifically, the neutrality legislation in effect aided the Fascist dictators—Italy's Benito Mussolini when applied to the Italo-Ethiopian War and Spain's Francisco Franco when applied to the Spanish Civil War. The legislation also tended, at least in the first of these cases, to undercut possible peacekeeping actions by the League of Nations.
Since even most isolationists agreed that the victories of the Italian and Spanish fascists were less desirable from the American viewpoint than were other possible outcomes, the wisdom of a policy that contributed to such a result came increasingly to be questioned. "In the long run," the Socialist Party leader Norman Thomas, a staunch isolationist and an original proponent of neutrality legislation, told President Roosevelt in December 1936, "it is not peace for the world, even for America which will be served by applying to the Spanish rebellion a general principle which should be asserted more rigorously than is yet the case in Congressional legislation concerning neutrality in international law." As a socialist who supported the elected government of Spain and abhorred Franco, Thomas was caught in a dilemma that could not be resolved in isolationist terms.
The two events that destroyed the rationale for American isolationism altogether were the fall of France in June 1940 and the attack on Pearl Harbor in December of the following year. The defeat of France by a seemingly invincible Germany created a profound sense of insecurity in the United States. It raised fears not only of an Axis victory but also of a direct attack on this country in the event, now deemed possible, that the British fleet would either be destroyed or captured. The founding of the Committee to Defend America by Aiding the Allies, the first influential interventionist organization, was a direct result of this fear, and the success of that organization produced the establishment of the America First Committee, the last stronghold of the embattled and soon outnumbered isolationists.
The attack on Pearl Harbor, in its turn, graphically demonstrated the vulnerability of American territory to foreign aggressors. Under these circumstances cooperation and even alliance with others to forestall further danger seemed dictated by prudence and common sense. "In my own mind," one of the most outspoken and influential of the congressional isolationists, Senator Arthur H. Vandenberg of Michigan, confided in his diary some time after the event, "my convictions regarding international cooperation and collective security for peace took firm form on the afternoon of the Pearl Harbor attack. That day ended isolationism for any realist."