Thus, when World War I broke out, although all parties made some play with the need to maintain or protect the balance of power (which, of course, they interpreted variously), none of them could argue that governments, or princes, were behaving in the way that one would expect. German apologists had to contend that Germany was surrounded by malevolent foes and that the survival of Germany was at stake. The allies had to contend not merely that Germany was too powerful for comfort, but that German militarism threatened a European civilization that would otherwise be peaceable. The argument, in short, could not be cast in terms of the balance of power.
Americans were presented with a dilemma. It was not, in the first instance, a dilemma of policy. Clearly the United States was not immediately threatened. The great growth of American power during the nineteenth century, if it made the policy of fortress America less necessary, made it no less appealing. It was hard to argue that the victor in the European war, whatever the outcome, would turn on the United States. Americans were therefore forced toward moral judgments about the merits of the war. Some indeed argued that what was going on was an old-fashioned struggle for the balance of power, of a sort that revealed how politically backward even the most advanced European states were, and of a sort with which the United States had no concern. Others accepted the argument that German militarism was the root of the trouble. Historians will long continue to debate the causes that finally brought the United States into the war, and their merits, but it is clear that no balance of power argument would have sufficed. A balance of power argument would have kept the United States neutral. (With the advantage of hindsight it might be argued that since the United States was the beneficiary of a balance of power in Europe likely to be upset, the proper American course was to intervene delicately to tip the balance back to the point at which it had been—and no more. Yet because the balance was bound to shift, war or no war, as the whole history of Europe in the 1920s and 1930s was to show, that kind of intervention could not have been temporary and would have required a degree of anxious vigilance over the long term, which could have been neither sustained nor justified.) Neutrality, defended on grounds of self-interest and its morality, or intervention, defended on moral grounds, were the only serious alternatives and the only alternatives debated.
The decision for war was President Woodrow Wilson's, and in taking it he was much moved by the realization that if the United States did not participate in the war, it would have no voice in the settlement that followed it. As part of the settlement Wilson was determined to establish an international concert—the League of Nations—which would bring about a better world order. Wilson's hostility to the balance of power was intense, and it was widely shared by Americans of his day. In an address at the Guildhall, London, on 29 December 1918, Wilson stated that
the center and characteristic of the old order was that unstable thing which we used to call the "balance of power"—a thing in which the balance was determined by the sword which was thrown in the one side or the other; a balance which was determined by the unstable equilibrium of competitive interests; a balance which was maintained by jealous watchfulness and an antagonism of interests which, though it was generally latent, was always deep-seated.
Wilson made an automatic connection between the balance of power and spheres of influence, to which he was equally opposed. That connection is characteristic of much American thinking on the subject; its consistency with adherence to the Monroe Doctrine is clearer to Americans than to others.
The approach of World War II presented Americans with a dilemma of a different sort. The Great Depression diverted attention from international affairs, but increasingly Americans could not avoid being drawn into efforts to mitigate both the depression itself and the political consequences that seemed to follow. The whole structure of reparations and war debts set up at Versailles would alone have required American involvement. The rise of aggressive regimes in Italy, Germany, and Japan, together with the long-cherished hope that they might be rendered more moderate by well-calculated economic concessions, or by democratic strength and solidarity, or a combination of these, ensured it. By contrast with the years before World War I, few Americans doubted on which side their sympathies lay. Whatever their fears of communism, the Soviet Union was quiescent, and the actions of the Nazis deprived their claim to be a bulwark against communism of all appeal. Secretary of State Cordell Hull (1933–1944) shared Wilson's dislike of the balance of power, and had learned it in the same school; but such views, although they became influential again later, were irrelevant in the 1930s, when it became ever clearer—certainly to President Franklin D. Roosevelt—that the important contest was not among rival states but between dictatorship and democracy.
Paradoxically, the desire of Europeans, especially the British, that the United States should become part of the balance of power—that the New World should be called in to redress the balance of the Old—and the fact that Americans had little doubt on which side their sympathies lay, did almost nothing to make policy decisions easier. The arguments, both within the American government and between Americans and British, are a fascinating and complex field, on which much work remains to be done. But in essence a dispute developed among the allies—even before the alliance was formed—over who should contribute how much to the common cause. The residue of American security, which was very great, together with well-founded doubts as to whether the interests of the United States might not be better served if some accommodation were reached in Europe without American intervention—doubts shared by some European statesmen, such as Neville Chamberlain—meant that American activity was diplomatically ineffective. A slow process of economic support for the Western democracies did begin, and might in time have drawn the United States into the war, but Hitler had the good sense to avoid the mistakes of his predecessors, and he was at great pains to avoid giving the United States an occasion for belligerency. That occasion was, of course, provided by Japan.
Some exponents of balance of power theory have argued that the theory requires that nations should match, if need be by war, any increase in a rival's power, actual or foreseen, even in the absence of any aggressive act. But all the evidence suggests that even when nations have adequate cause for war, they do not go to war unless they also have an occasion for war. The occasion, the indicator that the right moment for war has arrived, is vital. Of course, occasions for war can be manufactured when they are needed; but they are hard to manufacture, or even to identify, for a nation that disposes of such great reserves of security as the United States. One important argument is missing—the argument that if the nation does not fight now, it will be too late to fight tomorrow. It is that argument—with its corollary that opponents must be supposed to know how sensitive one's position is, and that therefore their threats are not accidental but evidence of real intention—which identifies most clearly the occasion for war. At Pearl Harbor, in 1941, the Japanese faced the United States with an affront such as no nation could possibly let pass. The Germans had been most careful to avoid an affront. (In World War I, on the other hand, when by reviving their unrestricted submarine campaign they deliberately took the risk of American intervention, a good many Americans could still be found to argue that the affront was not great enough to justify war in the absence of a real threat. The cause of neutral rights and of democracy had to be invoked.)
Just as a nation needs a signal to begin a war, so it needs a signal to stop, and that signal is often even harder to give or to detect. Because statesmen in the modern world are seldom wholly cynical, they commonly feel that war has been forced on them. As a war continues, they begin to raise their demands to include compensation for losses incurred. It is therefore hard to identify the point at which agreement for a truce can be reached, short of the final defeat of one side. Every success by either side leads it to think that final victory may be possible; every defeat, that this is not the moment to negotiate. It is the intellectual difficulty of translating the theory of the balance of power into a workable policy in a specific situation that, more than anything else, ensures that this theory is seldom of use when the time comes for negotiation.
These generalizations are supported by American practice in two world wars, yet American practice was not different from that of any other nation. Neither Britain nor France paid any special heed to the balance of power during either war. No way could be found of ending either war without the complete defeat of one side. After each war the recourse was not to some restored balance, but to a congress system. The experience of the League of Nations suggested to the allies in 1945 that no security structure was worth anything unless the great powers agreed, and that the right of veto might as well be formally accepted. If the five powers were not in agreement, the hope was at best for stalemate, by the agreed inactivity of four if one stood out. As always at the end of a war, what was in people's minds was peace, rather than either liberty or justice.