The horrific terrorist attacks yesterday in New York and Washington will rank as one of the greatest calamities in American history, and will confront the United States with one of its most demanding challenges. Not since Dec. 7, 1941, has the U.S. homeland sustained such an aggression. The nation responded then without panic but with iron determination to defend itself and punish the aggressors. The response today must be as decisive—to the mass murderers who planned and carried out the attack, and to any nation or nations that gave them shelter and encouragement.

It will be days or weeks before we can know the particulars of the death and destruction at the World Trade Center, at the Pentagon and in Pennsylvania, and absorb the magnitude of the losses; but what we know already is shocking, wrenching and infuriating. Thousands of American civilians—travelers, businessmen, tourists, government employees, children— were systematically targeted and slaughtered out of the blue sky of a late summer morning by an enemy that did not, and probably will not, willingly identify itself. In the days to come America must do what it can to ease the suffering of those who have been injured, comfort the families of those who were killed and quickly repair or clear away the physical damage. The Bush administration must move aggressively, as it did yesterday, to defend the country against possible follow-up attacks. And with soberness and determination, the nation must prepare itself to fight its first war of the new century—one that will begin with identifying and punishing the authors of yesterday’s mass murder, but one that must continue until the sources of support for the terrorists have been eliminated and the country’s defenses against such unconventional warfare decisively strengthened.

The challenge today is in many ways different from that of 60 years ago, and in some ways more complex. The attack at Pearl Harbor targeted military forces, not civilians, and when it was over the United States knew who the aggressors were and where to find them. All we know for sure about the enemy that struck yesterday is what the terrible wreckage in New York and Washington tells us: that this was an adversary capable of meticulously planning and executing a large-scale attack, one that draws on good intelligence and abundant resources. It is an enemy that has proven that it has the ability to penetrate U.S. homeland defenses, perhaps more readily than any the country has faced in modern times. And though it may have no single fixed address, it probably has the support or complicity of one or more foreign governments.

If the enemy is more elusive, yesterday’s attacks were not, or should not have been, as unexpected as was Pearl Harbor. The United States for years has been fighting a low-grade war against terrorists, and for years counterterrorism experts and military planners have been warning of the possibility of a massive strike against U.S. domestic targets. Some earlier attempts—including a previous plot to topple the World Trade Center—were narrowly averted. Steps have been taken in recent years to tighten airport and border security, and the FBI and CIA have mounted broad efforts to identify and uproot terrorist networks both at home and abroad. A few terrorists were apprehended and put on trial; a couple of cruise missile strikes have been launched; the networks of one leading suspect in yesterday’s attacks, Osama bin Laden, were said to have been seriously disrupted.

But the terrible message of Sept. 11 is that these steps fell far short; the nation’s commitment was not enough. Despite the increased airport security, the attackers managed to hijack four large airliners from three major airports— at Boston, Newark and Dulles—almost simultaneously, and flew one of them into the Pentagon’s restricted airspace apparently unchallenged. More broadly, an attack that must have required extensive preparations and a substantial support network appears to have gone entirely undetected by the FBI and intelligence community. These are large failings, the causes of which will have to be meticulously identified and remedied.

The challenge ahead will require strengthening U.S. defenses and intelligence at home in ways consistent with American values. Not just foreign embassies and military bases but also domestic airports and other civilian targets must be better defended. At the same time the country cannot allow terrorists to alter the fundamental openness of U.S. society or the government’s respect for civil liberties. Americans will have to make sacrifices that a state of war requires, such as accepting greater inconvenience in public places. They may also need to acquire some of the civic alertness that other open societies, such as Israel or Ireland, have cultivated.

A state of war also means a national commitment, nurtured by bipartisan cooperation in Washington, to attack and defeat the country’s enemies. This means more than merely tracking down and arresting individual suspects, as has been done before. With time, it is likely that it will be possible to identify the larger network behind the attack; it should also be evident where it is based or obtains support. In the past the United States has shied away from squarely confronting regimes that were linked to terrorist attacks against Americans— such as Iran in the case of the 1996 Khobar towers bombing in Saudi Arabia, or Afghanistan in the 1998 bombings of the U.S. embassies in Kenya and Tanzania by Osama bin Laden’s network. It can no longer afford to do so. Instead, it must seek to assemble an international alliance to identify and eliminate all sources of support for the terrorist networks that would wage war on the United States. If necessary, it must act alone. There can be no greater purpose to foreign and defense policy in the coming years.

Though the circumstances are different, what President Franklin D. Roosevelt said after Dec. 7 in Pearl Harbor, “a date which will live in infamy,” applies to Sept. 11 just as well. “Always will we remember the character of the onslaught against us,” Mr. Roosevelt said. “No matter how long it may take us to overcome this premeditated invasion, the American people in their righteous might will win through to absolute victory. I believe I interpret the will of the Congress and of the people when I assert that we will not only defend ourselves to the uttermost, but will make very certain that this form of treachery shall never endanger us again.”