All wars ultimately are competitions for power, but knowing that gets you no closer to understanding them.

All wars begin for specific causes, but knowing this, too, brings you no closer to understanding them. In 1946, at a beautiful society wedding in London, a Tory MP remarked to Lady "Emerald" Cunard how quickly life had returned to normal. "After all," he said, gesturing to the crowded room, "this is what we have been fighting for."

"What," she replied, "are they all Poles?"

The MP was closer to the truth. Germany had gone to war to expand its power, to impose its will on other nations; Britain had gone to war to protect itself from imposition. Few people held foremost in their minds at the war's end that five years of horror had been precipitated by the British treaty commitment to defend Poland from its totalitarian neighbor to the west.

In fact, part of the cost of victory to Britain was seeing Poland given up at war's end to be mauled by its totalitarian neighbor to the east. Churchill bitterly rued that fate and worked to stop it, but he had little power to do so and few others outside Poland regarded the situation with much concern.

To comprehend wars, some intermediate range of vision works best. The Greeks, as usual, discovered the golden mean. Thucydides, who was both a historian and a veteran, wrote that people go to war for "fear, profit, and honor" (deos, ophelia, and timē). It's almost exactly the same formula Jefferson hit on more than 2,100 years later when he wrote that the American rebels about to wage war on Britain, "mutually pledge to each other our Lives, our Fortunes and our sacred Honor."

Profit and fear seem pretty straightforward. People go to war out of a sense of fear of becoming isolated, impotent, and dominated by adversaries. The search for security inevitably is a search for power. Modern European nations, outwardly pacifistic, try to bind the greater power of America in international laws and protocols. In doing so, they enhance their own. All the while they denounce American power, they play the game as well, in their own way.

Thucydides' dictum was affirmed for modern times in Donald Kagan's "On the Origins of War and the Preservation of Peace" (1995). Kagan, with an eye to the complexities of modern international relations, translates ophelia with the broad and politic term "interest," but don't be fooled. The Greek also can be translated as "profit, advantage, benefit," and in some cases "spoil, booty."

But what of honor? It seems a quaint notion today. Yet Kagan says that, of the three, honor perhaps is the most urgent motive.

If we take honor to mean fame, glory, renown, or splendor, it may appear applicable only to an earlier time. If, however, we understand its significance as deference, esteem, just due, regard, respect, or prestige we will find it an important motive of nations in the modern world as well. Honor, in these senses, is desirable in itself, but it also has practical importance in the competition for power. When it is on the wane, so, too, is the power of the state losing it, and the reverse is also true. Power and honor have a reciprocal relationship. It is obvious that when a state's power grows, the deference and respect in which it is held are likely to grow as well. But the opposite is also true: even when its material power appears to remain the same, it really declines if in some manner these attitudes toward it change. This happens most frequently when a state is seen to lack the will to use its material power.

The whole modern (post-1944) history of France is incomprehensible without reference to honor, specifically to the French need to maintain the fiction that their country remains in the first tier of world powers. And this is understandable; to accept demotion would be an irrecoverable slight to honor and a consequent reduction in France's power to protect itself, or influence world affairs. France will in one moment taunt the United States, and the next turn around and waggle its nuclear fig leaf in the direction of the Teheran mullahs. As de Gaulle once put it, "France cannot be France without grandeur."

Marxist anti-war rhetoric obscured the importance of honor in America's failed bid to create and sustain an independent South Vietnam. A communist victory in South Vietnam would made no dent in America's material interests, nor would it make American measurably less secure. Johnson and Kennedy both knew this. But once committed, our honor was at stake. Bin Laden and his ilk certainly understand this; they continually taunt America's allies in the Middle East with the image of America going back on its word and abandoning its ally in Southeast Asia.

Some American historians, but too few, have noted the central role of honor in sparking the American Civil War. The North, rabidly anti-black and contemptuous of the runaway slaves that seeped into its cities and raised the crime rate, nonetheless irrationally resisted the Fugitive Slave Laws from the 1830s through the 1850s, and risked open conflict with the South to protect its legal institutions from being forced to participate in slave recaptures. What but honor motivated that? And the South felt its honor taunted and reviled constantly by the Northern abolitionist politicians. What was the whole Civil War but the Brooks-Sumner caning incident writ large?

"The surrender of life is nothing to sinking down into acknowledgment of inferiority," John C. Calhoun once wrote. De Gaulle, if he had read that, would have nodded in agreement.

America's wars in the 19th century fall into the category of causes compiled by Thucydides. The felt need to preserve national honor played key roles in the War of 1812 and the war against the Barbary Coast pirates. The Mexican War had little to do with honor, but a great deal to do with ophelia -- booty. This also motivated the War of 1812, in part, though the outcome obscures the fact that the War Hawks of that year considered Canada easy pickings.

Something new emerged in the 20th century in America's wars, however: some new motive quality not anticipated by Thucydides. It is rare in history; in trying to find earlier examples, I can only think of France after the Revolution. You may call this quality "muscular benevolence," or "violent charity" or "idealism."

It emerged first, I think, in the Spanish-American war, though that war also featured aspects of honor ("Remember the Maine") and profit. But intermingled with the various motives was a desire to drive the last vestiges of the last colonial powers from the New World, and set people free. If nothing else, the war revealed the potential of the generally isolationist and anti-imperial American people to be persuaded that freedom was a gift they had the ability and duty to offer to other peoples. That the war ended in the tragedy of the Philippines occupation was an early lesson in the gulf between ideals and realities.

With that war, America emerged on the world scene. Given America's surging military power, and the nature of modern industrial warfare, America's isolationism was doomed. There was no turning back. Europeans recognized this before Americans did. French commentator André Tardieu noted in 1908 that the United States "is seated at the table where the great game is played, and it cannot leave it."

Americans clung to the isolationism that was their nation's unique legacy because America was, well, isolated. Geography had granted us the remarkable luxury of even fighting a civil war without foreign interference. But by the early 20th century America's leaders began to realize that day was past. Two answers emerged to what American power might become on the international stage.

One was represented by Teddy Roosevelt. It was self-interest, realistic, unromantic; the foreign policy of the Panama Canal, the Great White Fleet, and the Big Stick. The United States would join the game and play by the rules already set by the European powers.

But such a strategy failed to convince essentially isolationist Americans to venture out into the cold, ugly world. It was an alien policy, against all the deeply absorbed warnings of Washington and other Founders against "foreign entanglements."

Woodrow Wilson, who embodied the alternative, also eventually realized that America could not hold aloof from the world, especially as World War I unleashed the horrific power of modern industrial warfare. But he based his foreign policy on American ideals and values. Instead of denying American exceptionalism, he embraced it and built it into his policy. Freedom and democracy were the gift of the gods to the American people, to be cultivated, and to be shared.

Explicitly eschewing selfish national interest, he sought peace without victory and international harmony in a compact of nations. I've written extensively about Wilsonian visions, and America in the Great War, here. Europeans were openly contemptuous of his messianic zeal. And America, too, seemed unconvinced. After the war, the nation returned to its false isolationism.

The two visions of American power slumbered for a generation. When America went to war again, in 1941, it emerged at the pinnacle of the international power structure. Yet even then it tried to pull back into the 19th century cocoon. Wives of servicemen organized Bring Back Daddy clubs, and there was talk of impeachment in 1946 if Truman didn't demobilize fast enough.

Reality soon intruded and denied this fantasy. But it pulls strongly at us. In 1989, as the Cold War ended, the dream of isolationism again beckoned.

The growing dependence of America -- and especially of its key allies and trading partners -- on foreign oil put the material needs of ophelia at the center of our debates as they never had been before. Fear -- deos -- was the deterrent in the Cold War, but with the rise of Islamist terrorism and the spread of small but deadly weapons it has become the pretext for aggressive action. Honor -- the timē of Thucydides -- plays the central role it always has taken. And, as more thoughtful critics have noted, the Wilsonian vision allows America to enjoy the paradox of flexing its muscle while maintaining its faith in its own innocence.

American foreign policy and use of military force in the last 50 years can be understood in terms of the balance of these three ancient causes -- profit, fear, and honor -- along with the tensions between isolationism and Wilsonian idealism. The policies of Nixon and Kissinger, for instance, featured the cynical realism of Teddy Roosevelt wrapped in the noble mantle of Wilson.

Why did America go to war against Saddam Hussein in 2003? Clearly there was fear. We debate endlessly and frivolously how much the fear turned out to be justified. But the fact remains, no amount of intelligence about Iraq's weapons and controllinging force on Saddam's intentions would have been flawless so long as he remained in power. And in that gap between what we know and what we suspect, always would have been fear.

Clearly there was an awareness of the "profit" -- the riches of Iraq's oil. Despite the angry denunciations of "blood for oil," however, I think the worst the Americans can be accused of is intending to use Iraq's oil to pay for the war and the reconstruction, which hardly amounts to a crime against humanity. It didn't work, anyhow.

And clearly there was a question of national honor. Every day Saddam lived to murder and mock, to rape and preen, was felt as an affront to America. It must have been an especial affront to George W. Bush, Colin Powell, and many others in the administration who had failed to topple the dictator in 1991.

There was, indisputably, the Wilsonian ideal, embodied in the "neo-cons" and the liberal interventionists. There are Little Roosevelts in the mix, too, grumbling about the administration's failure to grasp the hard truths of Realpolitik. And there are triangluations of the positions: "high-minded realists," for instance, who stand apart from the self-appointed champions of global democracy but who recognize that a stated preference for liberty and justice can be a useful foreign policy tool in the fight against global terror.

And there is the primeval American isolationism, still clinging to 19th century fantasies, as an undertone in the world of the Cindy Sheehans and Michael Moores.

The world didn't change on Sept. 11, 2001, as much as we think it did. This is familiar terrain.

Here's a snippet from an interview with Joel Stein, the infamous "do not support our troops" newspaper columnist:

HH: Do you honor the service that their son did?

JS: To honor the service their son...now this is a dumb question, but what do you mean by honor? That's a word you keep using. I'm not entirely...maybe that's my problem. But I'm not entirely sure what you're...