America must awaken to the seriousness of military power: So argues Donald Kagan, magisterial author of a four-volume history of the Peloponnesian War, along with his son Frederick, who teaches at West Point. The transparent organization of their book makes it easy to follow. A succession of brilliant insights make it, along with Walter McDougall’s Promised Land, Crusader State, the best foreign policy book of the post-Cold War period. Nevertheless, the book’s explanations of and remedies for military unseriousness are not what one would expect from a great student of Thucydides.

The book contends that Britain in the 1920s and ’30s, and the U.S. in the 1990s, failed to shape the international environment in their own interest as “satisfied powers” because they did not husband enough military power, nor commit it to prevent the growth of forces that upset the international order. The first half of the book is a detailed analysis of British policy with forward glances to our time, while the second takes us from the fall of the Soviet Union to the Kosovo war through the eyes of various U.S. policy makers and with plenty of comparisons to Britain’s earlier failures. The book does not argue that America is now facing the equivalent of a Germany going Nazi, or a Japan invading Manchuria, leading to a repeat of World War II. Rather, it shows that U.S. policy in the 1990s resembles British policy in the ’20s. It argues that such policies of engagement combined with weakness naturally produce what Machiavelli called the deadly combination of hatred and contempt.

One year after the armistice of November 11, 1918, the British army had shrunk from 3.5 million men to 375,000. Even as the Versailles treaty was being negotiated, Britain was losing the military capacity to consummate victory and hence to enforce any peace on Germany. Had the treaty been milder, Germany might not have caused trouble even for a weak Britain. Had Britain been much stronger, Germany’s rage against an even harsher treaty would not have been dangerous. But Versailles was as harsh as Britain was weak. Versailles also left the rest of the world disorganized. If Britain had had no interest in an orderly world, its inability to bring order would not have mattered. But Britain had fought the war in part for the Wilsonian goal of a new world order. When the rest of the world noticed Britain’s impotence, that goal too became impossible. Iraqis nearly inflicted on Britain the kind of defeat that would have chased it out of the Arab world. Britain held on to this part of the empire by further mortgaging its hold on India. Mustafa Kemal’s rebels raged against the straits treaty that Britain had imposed on the Ottoman Sultan, overthrew him, and nearly overran the British garrison guarding the straits. Britain saved face by calling on Greek forces and giving in to most of the Turkish demands. Mussolini invaded Corfu, and Britain bought him off.

British leaders convinced themselves nevertheless that the shadow of military power was enough to manage the new world order. They based their military planning on “the ten year rule,” the formal assumption that Britain would not face what nowadays we call a “peer competitor” for at least ten years, and that it would take at least ten years from detection for such a peer competitor to arise. Hence they got into the habit of building military forces not to meet military or political goals but rather to meet a priori budget ceilings. The book even quotes Winston Churchill in the chorus of support for that ruinous idea.

British leaders thought they could manage the new world order with little military force because it would be secured by “an international organization of states, the League of Nations,” or at least by alliances of likeminded states. But it misunderstood the league, and could not bring itself to practice orthodox alliance politics. The most disastrous instance of attempting to straddle the two was the crisis over Mussolini’s 1935 invasion of Abyssinia. Mussolini had joined France in opposing the growth of German power while Britain was refusing to do so. Britain had no intention of making war to stop Mussolini, and no one had any illusions that sanctions would do the job. Moreover, everyone knew that Franco-British sanctions would leave Italy with only one possible ally: Germany. Nevertheless, Britain asked a worried France to join in sanctions against Italy in the name of the League without giving France any more support against Germany.

British leaders behaved fecklessly because they believed that the public would not support vigorous policies, and by so doing further confused the public. Quotes from opinion leaders at the time who wanted peace but would not consider war to secure it foreshadow similar sentiments in America today.

Even before the Soviet Union had died, American politicians were vying to cut U.S. military power without reference to any but the needs of the day while assuring the public that what was left would be enough for tomorrow. The Gulf War, the book argues, came because Iraq mistakenly but reasonably supposed that the U.S. was beyond supporting its foreign policy with military force. The first Bush Administration was potent enough to go to war, but proved its limits by throwing away the victory. Then, it proved its ambivalence again. It formulated a sensible intellectual framework for a “base” U.S. military force to support foreign policy for a long run that included war: Defense Secretary Cheney explained to Congress that “there always is” a next war. But then, it undercut its own case by asking for military forces too small to do the jobs it had outlined. This emboldened opponents, led by Defense Secretary-to-be Les Aspin, to cut U.S. forces down to arbitrary budget numbers on the basis of a “Bottom-Up Review,” a homegrown version of the “Ten Year Rule.”

On the basis of much-reduced military forces, the Clinton Administration nevertheless tried to take a hand in more of the world’s affairs than had its predecessor. The resulting engagements in Somalia, Haiti, Bosnia, and Kosovo, the expansion of NATO’s mission to Eastern Europe, not to mention movement in the Middle East and East Asia, amounted to a combination of bluff abroad and pretense of success at home—the equivalent of writing checks with insufficient funds. One of the book’s virtues is its dissection of the disastrousness of half-measures. One would expect no less from someone steeped in Thucydides’ lesson of the dire effects on the Sicilian expedition of choosing the “middle” strategy. Inevitably, as America’s words got louder and its stick got whittled down, as its forces became smaller, its equipment older, its personnel demoralized and spread thinner around the world, as its enemies prospered, it invited contempt. In short, the book abounds in lessons on policy-making.

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Much less satisfactory is the book’s analysis of why this has happened and what might be done about it. The book indicts Britain’s “war weariness” and its contemporary U.S. equivalent, the “Vietnam Syndrome.” Above all, it blames politicians for not striving against such domestic sentiments. It says that America, properly led, would be willing to “sacrifice her sons and daughters.” This formulation raises three questions: Why did the “weariness” or “syndrome” become problems in the first place? How and why did British and why do American leaders lead so badly, and of what would proper leadership consist? And for what should we Americans hazard our lives?

Casualties in World War I were demographically significant. Casualties in Vietnam were not. In the Gulf War, casualties were practically nonexistent. World War I and the Gulf War were victories, Vietnam a defeat. Nevertheless, all these wars turned the British and American people respectively away from muscular foreign policy. The reason has less to do with the quantity of casualties or the verdict of war than it does with the judgment that none of these wars was worth the sacrifice. Why not? Because each one in its own way was ill-conceived. For all that the people got from these wars, they would have done better to stay home. Especially galling was the disproportion between the avowed objectives, in each case having something to do with a new world order, and the result. Pace the Kagans, “weariness” and “syndrome” are actually reasonable responses to incompetent statesmanship.

Our statesmen failed because they got into the habit of promising escape from the timeless threat of war, to be achieved by some sort of organizational stroke—or at any rate, without war. The book does not realize that the League of Nations and other forms of collective security are attractive to peoples not as opportunities to take a merely proportionate part in policing the world, but rather as opportunities to escape responsibility for their own safety. The book’s references to the League of Nations (and to a lesser extent to the “new world order” of the 1990s) do not recognize that they were self-contradictory concepts: communities of most of the world’s nations, though not all, that had now sworn off war, and yet were expected to make traditional preparations for allied war against likely aggressors. Peoples can sense their leaders’ self-contradictions and fraud. This will not improve until our statesmen learn to look war in the face again.

The first reasonable question about any war is: What for? Why should I get myself killed? The answer had better be good. This leads us to the book’s main practical purpose and its greatest weakness. The book is an apology for what has come to be known in American discourse as “interventionism.” Yes, on several occasions the authors write that they do not think Britain should have been or America should be the world’s policeman. Elsewhere, they write that they advocate only “the maintenance of a balance of security in all regions of vital interest to the United States.” But that turns out to be pretty much the world. And on the next to last page they write: “It is the experience of the last century and the one before it that to preserve the peace of the world requires a ‘police force.'” Does the book mean that we should make war everywhere? Not exactly. It states repeatedly that in most cases “intermediate measures” will suffice. But the reader will be hard pressed to distinguish between “intermediate measures” and “half-measures.” Hence the book answers the question “For what should I risk my life?” with: To police the world, because we can’t have peace anywhere unless we have it everywhere. That is not good enough.

The book does not deal with a problem besetting U.S. national security that arguably underlies all the ones it mentions, namely that American elites—notably the likes of Donald Kagan’s Yale colleagues and students—have opted out of military service. If our elites could be persuaded as a matter of policy to try policing the world, they would seek to do it with hired hands. What would happen then would be more like a book by Edward Gibbon.