One of the greatest virtues of democracies is that they do not go to war easily. That is true even when they have been subjected to attack; it is even more so when they are challenged to use force without a proximate act of aggression. The case of Iraq is no exception: In the United States, where a substantial majority of the public supports military action to disarm Saddam Hussein, there have nevertheless been large and passionate demonstrations in opposition. In Europe, where the sense of danger from Iraq and weapons of mass destruction is less, big majorities oppose a U.S.-led campaign, and protest rallies last weekend attracted immense crowds. We continue to believe that war with Iraq will be necessary, unless there is a dramatic change in Baghdad. But if this clamor of opposition is making it harder for the Bush administration and its allies to go forward, that is probably for the better. In a democracy, before any bombs fall, governments should be challenged again and again to explain why force is necessary, why the alternatives are not acceptable and why the outcome will be worth the always terrible costs. The continuing dissent both here and abroad should inspire President Bush to make his case more clearly and strongly.

But it should not paralyze him or the several dozen other leaders who now appear ready to stand with him. Democratic opposition can prevent irrational belligerence. But leaders of democracies sometimes must make painful and even unpopular choices, whether to raise taxes or to send young men and women into battle. The greatest vulnerability of democracies comes when their leaders falter in the face of such choices, as in the Europe of the 1930s. In contrast, the ability of American leaders to persevere with the costly and often controversial measures needed to contain the Soviet Union eventually yielded a triumph of freedom over totalitarianism.

In the case of Iraq, the functioning of American democracy has been pretty straightforward. President Bush has been respectful of opponents, at least at home, as he should be on such a momentous issue. He accepted advice to seek congressional approval and to make his case before the United Nations. His policy is supported by close to 70 percent of the country, a remarkable figure in a nation that split down the middle in the last presidential election. Some complain that Congress, or the Democratic Party, has been fainthearted in checking the president's preparations for war. But even most opponents in Congress share the country's sense of the threat from Saddam Hussein. Their questions are mostly about tactics, about preserving alliances and about postwar plans -- all issues that the administration could, and should, address better.

The case in Europe is more complex. A handful of leaders, including French President Jacques Chirac and German Chancellor Gerhard Schroeder, have chosen to ride a wave of antiwar and increasingly anti-American opinion. Remarkably, however, a far larger number -- 18 and counting -- are looking past the polls and reasserting their support for the United States. Maybe Mr. Chirac and Mr. Schroeder really believe it is wrong to forcibly disarm and depose Saddam Hussein; maybe they hope to use the sentiment in the streets for their own political ends. Either way, the path they have chosen isn't the hard one. The toughest political act in the world today is being performed by British Prime Minister Tony Blair, Spanish Prime Minister Jose Maria Aznar and their allies. Mr. Blair and Mr. Aznar watched the protests that filled their capitals last weekend, acknowledged the message, then called Mr. Bush to say they would not back down before Saddam Hussein. Theirs is a stand on principle, a recognition that this is one of those rare moments when leaders must insist on the tough course that their voters would rather avoid. They know they will be held accountable for the results. If Iraq is successfully liberated and disarmed in the coming months, its people will have Mr. Blair and Mr. Aznar to thank, along with Mr. Bush. If Saddam Hussein survives in power, he, too, will know which leaders helped preserve him.