All wars end eventually. But some don't end when and where you think they will.

Take the civil war in Syria. When that conflict ends, the forces opposing Assad will move elsewhere, including to Europe. We may not want to intervene in Syria, but Syria is likely to intervene in us.

In the Middle East, strong governments, like Iran's, are often bad. But an absence of government is no better: Islamist radicals happily fill the vacuum. With a weak regime in Egypt, a weaker one in Libya, and the near-collapse of Syria, these vacuums are multiplying.

The radicals have one weakness: They try to control territory. They tried earlier this year in Mali. Now they are trying in Syria. That makes them vulnerable, because radical Islamists dislike Western-style states. To them, national boundaries are merely arbitrary lines: Religion is the true divider of humanity.

The radicals are superb at overthrowing regimes, but less good at building governing institutions that can mobilize power. This paradox of radical Islam means that, sooner or later, its adherents lose control and flow elsewhere, looking for another regime to drag down.

On Tuesday, 11 rebel groups in Syria declared allegiance to an al-Qaida affiliate and demanded "Shariah law for the country." They will not simply quit when they lose Syria.

Jordan and other countries in the region are most at risk when the fighters move on. But several hundred of these radicals reportedly come from Europe, and they will likely return. Others will move there illegally or as refugees. These fighters will pose a direct threat to Europe, and an indirect one to the United States.

Robin Simcox, a leading British authority on al-Qaida attacks, recently testified to Congress on this danger. He offered one sliver of comfort: To date, most attacks in the West have been by terrorists who trained abroad and came back with a target and a plan.

But Simcox also pointed out that Syria poses dangers apart from direct attacks. The fighting there will give inspiration to a new generation of radicals, just as the Taliban did in the 1990s. Others will blame us for failing to intervene in Syria and, like the Tsarnaev brothers in Boston, mix Islamist radicalism and Western anti-Semitism into a noxious home brew.

Of course, the flow of radicals is not one way. Two or three Americans and a British subject are said to have participated in the recent massacre in Kenya. Participating in mass slaughter marks a dangerous escalation from earlier reports indicating that approximately 40 U.S. radicals have joined al-Qaida affiliates in Somalia.

We need to recognize that jihadism is, indeed, a religiously motivated ideology. If we cannot be honest about why its proponents fight, we cannot hope to understand them.

We should also be honest about the prominence of radicalism among the Syrian rebels. Secretary of State John Kerry's assertion that the rebels there were "increasingly . . . more defined . . . by moderation" was obviously aimed only at justifying U.S. intervention.

We also need better cooperation from our allies. The European Parliament has repeatedly held up efforts to share passenger data on incoming flights to the United States, a common-sense measure that would make us all more secure. We also need to be a supportive ally ourselves to Kenya, which has led the fight against al-Shabab in Somalia, and has now been attacked in retaliation.

Above all, we need to recognize a fact: No matter how much the president says that al-Qaida is on "a path to defeat," our enemies do not see it that way. We may not want to be interested in them, but they are going to stay interested in us.

- Ted R. Bromund, Ph.D. is a Senior Research Fellow in Anglo-American Relations at the Heritage Foundation.