La Belle Equipe, the last of the cafes and bars destroyed in the Paris terrorist attacks in November to reopen its doors, finally did so Monday. Nineteen people had been killed on the cafe’s premises, but Parisians flocked to its reopening as a way to celebrate victory over fear and those who would inflict it.

The next morning, though, terrorists struck again, this time in Brussels, some 200 miles away. Three explosions — at the airport and on the subway — killed more than 30 people. ISIS, the same Islamic terror group that had perpetrated the Paris attacks four months earlier, killing 130 innocent people, claimed responsibility for the Belgium attack as well.

In the wake of the attacks, European leaders and President Obama quickly stressed their unity against a common enemy, a very different message than that communicated a day earlier by Donald Trump. In a meeting with the Washington Post editorial board, the GOP frontrunner had argued that the United States needs to “rethink” its commitment to NATO, suggesting it is no longer essential to U.S. interests.

That would be a substantial mistake, as the terror attacks demonstrated. Over the past year, the United States and its allies in NATO and in the Persian Gulf have made significant progress in the military battle against ISIS in Iraq and Syria. The effort has significantly reduced the amount of territory held by ISIS in its so-called “caliphate,” the flow of recruits into the region has been largely shut down, and as financing gets tighter and their territory shrinks, ISIS fighters are reportedly abandoning the cause.

Clearly, the group does retain the ability to strike well outside its zone of control, taking advantage of the chaos that wracks the Middle East. But the airstrikes that continue to weaken ISIS’ command structure are possible only because they are being launched from U.S. bases in NATO countries such as Turkey and Italy. Any suggestion that the United States might withdraw from the fight, leaving Europe to fend for itself in this crisis, would be remarkably irresponsible and counterproductive.

“Ladies and gentlemen, we are defeating this enemy in Iraq and Syria,” CENTCOM commander Gen. Lloyd Austin testified to the Senate Armed Services Committee earlier this month. “We are pressuring (ISIS) on more fronts than at any other point in time since he marched into Mosul some 18 months ago. And we’re doing so by degrading the enemy’s military capability, by taking back territory, by diminishing his economic resources and by removing his senior leadership from the battlefield.”

But as Austin also warned, “military success will be lasting only if corresponding political progress is achieved in both Iraq and Syria.”

That ought to sound familiar. U.S. military leaders have issued some variation of that statement for almost 15 years now, from the initial invasion of Iraq through the surge and its aftermath, warning that military action alone cannot calm the chaos from which such threats emerge. And if overwhelming military force cannot solve it; nor can withdrawing into a self-protective shell, as if pretending the world cannot reach us.

As French President Francois Hollande noted Tuesday, this is a war, and “this war will be long.”