By launching armed attacks in Syria last weekend, the United States sent another warning to governments that refuse to stop terrorists who operate on their territory. U.S. President George W. Bush has made it clear that he will not stand idly by as terrorists target American troops, and some scholars insist he has the right to take action. But the real question is whether unilateral moves will solve the problem or make it worse. The record is not encouraging.

Convinced that the government in Damascus was ignoring charges that terrorists were using Syrian territory to enter Iraq, the U.S. last weekend sent four helicopters carrying special forces 6.5 km into Syria to disrupt the network. The assault targeted Badran Turki Hishan al-Mazidih, known as Abu Ghadiyah, who heads Syrian logistics for the insurgent group al-Qaida in Iraq. Reportedly the U.S. had intelligence that Abu Ghadiyah was preparing a terrorist attack in Iraq. The U.S. troops killed several men and reportedly seized some others. Syria countered that the attack was "a criminal and terrorist aggression" that killed seven civilians.

There is little doubt that insurgents use the 560-km border to cross from Syria into Iraq. At one point, it was estimated that more than 120 men entered Iraq each month via this route. Documents seized in a raid in 2007 on an al-Qaida safe house in Iraq revealed substantial details about the network that helped them cross the border and about its operations. There are, however, questions about Syrian government complicity, whether it is truly involved or merely ready to turn a blind eye.

Syria insists that it is doing all that it can. In fact, Western military leaders have recently praised Syrian efforts to halt the flow of insurgents, which has been cut to about 20 a month. Given that the government in Damascus is run by a minority Alawite sect, it has little reason to make common cause with the Sunnis who dominate al-Qaida; it blames al-Qaida for a summer bombing in the capital. Saddam Hussein also had reason to keep al-Qaida from operating in Iraq when he was in power.

But the Syrian government is also reluctant to alienate local tribes that are sympathetic to smugglers, and who rely on the payoffs from them to supplement their meager earnings. Syrian public outrage against U.S. policy and various incidents, such as whenever U.S. forces launch offensives against Sunnis within Iraq, also weaken Damascus' will to police its borders.

U.S. officials say their message is clear: Governments must respond to the threat on their territory or the U.S. will do it for them. This message has also been sent to Pakistan, where the U.S. has launched missile strikes against suspected Taliban and al-Qaida redoubts near the border with Afghanistan.

Iran is also paying attention to the U.S. strikes, as it is alleged to host camps that train foreign insurgents before they enter Iraq. But Washington should worry whether such unilateral actions will whip up domestic sentiment in Arab countries against it to a level that makes it difficult if not impossible for those governments to cooperate with the U.S. These may not be democracies, but they are not impervious to public opinion.

Some see in the moves a new manifestation of the "Bush doctrine." Unlike the preemption argument that was used to justify the invasion of Iraq, this new rationale asserts that the U.S. — or any government — has the right to strike specific targets that seek to do it harm.

Mr. Bush made his case in a speech to the United Nations General Assembly last month: "As sovereign states, we have an obligation to govern responsibly, and solve problems before they spill across borders. We have an obligation to prevent our territory from being used as a sanctuary for terrorism and proliferation and human trafficking and organized crime."

Worrisome though it may be, this notion does have precedents in international law. In particular, supporters point to Article 51 of the U.N. Charter, which gives all nations the right to individual or collective self-defense. This theory has justified similar state action in the past: Israel used it in 1976 during the Entebbe raid and Turkey has used it to justify pursuit of Kurdish rebels into Iraq's territory. Of course, such reasoning depends on genuine and accurate intelligence proving the existence of such a threat and it requires that the government "hosting" the terrorists be unable or unwilling to act on its own.

History has also demonstrated that such evidence is rarely as clear as it seems and judgments about who is or is not taking appropriate actions are invariably subjective. The U.S. may intend to spur its adversaries into action that reduces threats against it, but the more likely consequence is yet greater resistance to U.S. policy, making it even more difficult for Washington to rally support to its cause.

American frustration is understandable, but that must not justify actions that only make it harder for Washington to realize its ultimate objectives.