Americans are enamored with our own goodness. We like to think of ourselves as peace-loving, law-abiding, virtuous—a model to the world. "America has not started a war in this century," Newsweek proudly declared at the end of the last century, summarizing 100 years of warfare and encapsulating our belief in our purity. One reason that many people have qualms about the looming invasion of Iraq—in which the United States intends to strike first without an unambiguous casus belli—is that we imagine that we go to war only when provoked. As we debate commencing hostilities, it's worth reviewing our reasons for waging war in the past, for our retrospective judgments about those conflicts should influence how and whether we go to war today.

To give America its due: In those wars dearest to our national mythology—the American Revolution, the Civil War, World War I, World War II—we went to war for what still appear to be sound reasons. (Indeed, that may be why these wars have become so precious to our collective memory while more shameful wars are forgotten.) The American Revolution occurred only after a decade of increasingly burdensome taxation without representation by the English monarchy, and it was fought in the name of self-government. The casus belli was the British army's march upon a cache of colonists' arms in Concord—a martial act that deserved a martial response.

It's equally hard to gainsay the nobility of the fight to maintain the Union. Although unspeakably horrific—more Americans died in the Civil War than in all other conflicts combined—the war was needed to prevent the fracture of the nation and the spread of slavery. Again, we had clear provocation when the Confederate rebels fired on federal ships at Fort Sumter. So, too, with World War II. America's 1941 oil embargo against Japan (a response to Japanese aggression in Asia) ratcheted up tensions, but no one would claim that after Pearl Harbor the United States lacked just cause in declaring war on its attacker.

World War I lacks the grandeur of "the good war," and there remains a plausible case against American intervention. Indeed, the United States stayed neutral in Europe's combat until 1917, and we might have technically been able to abstain further. But Germany's provocations made neutrality increasingly untenable. First, the United States intercepted (via Great Britain) the Zimmerman Telegram, a note from Germany's foreign minister to his Mexican ambassador proposing an alliance against America. The telegram's publication prompted President Wilson to sever diplomatic ties with Germany, which then sank three U.S. merchant ships in the Atlantic, triggering our entry into the war. Had the United States declared war on the basis of the telegram, it might have been undertaking a pre-emptive war (which is significantly different from the preventive war that we're contemplating against Iraq. Click here to learn the difference). By waiting until attacked, Wilson won a greater measure of legitimacy for joining the combat.

To look at these examples is to find a creditable American moral track record. They would suggest the pending Iraq attack is unprecedented, that it violates our traditions. You could even bolster this case by throwing in some of our less noble conflicts—1812, Korea, Vietnam—which, at least initially, had defensible rationales. In the War of 1812, after all, the British were forcing American maritime merchants into service in their own sea battles with France. To turn this into a casus belli was controversial—the war sharply divided American public opinion—but not a sham contrivance.

Similarly, in Korea and Vietnam, the United States was coming to the aid of an ally attacked by Communist neighbors. That didn't necessarily make these interventions wise. Korea probably was, on balance, a just war, but it's hard to say the same for Vietnam—in which the United States was propping up an unpopular regime based on misguided domino-theory thinking and buttressed by a dishonestly secured congressional authorization. Over time, Americans rightly concluded that protecting Cold War credibility wasn't worth losing tens of thousands of young men (to say nothing of the devastation we wrought on the Vietnamese). Still, until 1968 or so, when the struggle became patently unwinnable, one could defend the U.S. presence in Indochina in good faith. The key question wasn't so much about whether we should have gone as whether we should stay.

That said, the United States has also deployed its troops to foreign lands for dubious reasons—sometimes reasons far less valid than disarming a genocidal, belligerent tyrant who is trying to build nuclear weapons. Americans have rarely evinced qualms, for example, about policing their own hemisphere. In 1823, President James Monroe declared that the United States, not Europe, should superintend the regimes of the West. Presidents ever since have used the Monroe Doctrine as license to invade banana republics—especially since Theodore Roosevelt, whose "Roosevelt Corollary" explicitly sanctioned military intervention south of the border.

Indeed, for a century, the United States has cheerfully effected "regime change"—that bloodless euphemism for killing or deposing other nations' leaders—in Nicaragua, Guatemala, the Dominican Republic, Grenada, up through our gratuitous 1989 invasion of Panama. In Friday's New York Times, Max Boot tries to justify some of these incursions as "pre-emptive," which they assuredly weren't; none of these regimes ever could have or would have harmed us, and our meddling in Latin America mainly served to substantiate the worst fears about Yanqui imperialists. Nonetheless, because of their relative proximity, smallness of scale, and lack of risk, these backyard wars elicited far less controversy—in the United States and Europe, that is—than has the upcoming war on Iraq because far less was at stake. They don't really provide precedent for the Iraq invasion.

More controversial than these quickie incursions are those many wars we started or exploited to gain land. It's hard to deny that European Americans, and the nation that they founded, "started" the wars with the Indians of North America. Although historians argue over who provoked whom in particular conflicts, overall it was the white man's persistent extension of the frontier in search of territory that foreclosed any chance of peaceful coexistence.

It's also hard to plead American innocence in the Mexican War of 1846-48. Mexico and the United States were at odds over the borders of Texas, which joined the Union in 1845. After a diplomatic snub, the United States sent troops to the disputed area. Mexico, considering itself invaded, attacked the American regiments, and the United States then declared war. In the peace treaty that eventually followed, the United States forced Mexico to cede not only Texas but the Western lands stretching to California—a vast booty for what had begun as a squabble over the southern swath of one state.

Finally, there's the morally suspect case of the Spanish-American War. When Cuban rebels rose up in 1895 against their Spanish colonizers, Americans sympathized with the Cubans, and many favored entering the war to help drive Spain out. But it took the 1898 explosion of an American ship, the Maine, which President McKinley had sent to Havana to protect Americans there, to bring the United States into combat. The blast, which killed 260 crew members, was probably caused by technical problems on the ship, but that wasn't evident at the time. The American public instantly became convinced of Spanish malevolence and demanded war. The United States invaded not just Cuba but the far-flung Philippines, also a Spanish colony. By the war's end, the United States had become a full-fledged imperial power, with rights to the Philippines, Guam, and Puerto Rico.

Unlike our great and good wars, the Spanish and Mexican conflicts are little remembered (despite entreaties to remember the Alamo and the Maine). The absence of a moral grounding—the realization over time that in each case the casus belli was fairly bogus—discredited the American enterprise and bolstered those who derided the nation as expansionist, imperialist, or genocidal. Yet alongside these inglorious examples, America also has a tradition of waging war for honorable reasons that it could offer to the world as legitimate grounds for making war. For these wars, we not only congratulate ourselves but also gain the affection of others. The current debate about war should address not only whether we go to war but also why: If and when we invade, we should do so not because we deem it justifiable but because we can show that it is just.

This article first appeared on Slate.com and is reprinted with permission of the author.