President Barack Obama has no choice now. If he wants to defeat the Islamic State, he’ll have to become a real war leader. He may not like it, but putting troops on the ground is the surest way to win this war.

That’s nothing new. Paris was horrific, but no game-changer. The game changed in 2014, when the Islamic State broke out from Syria, drove the Iraqi military from the field and proclaimed a worldwide caliphate.

The moment the terrorist group established a territorial state in Iraq, it posed two grave threats to the U.S. and its friends and allies.

A Sunni terrorist-controlled state in the heart of the Middle East was guaranteed to create friction that will lead, inevitably, to a larger regional conflict with far-reaching spillover consequences. The refugees flooding Europe and Moscow’s deepening of its direct involvement in the fighting in Syria foreshadow what lies ahead.

The second threat was transnational. With tens of thousands of foreign fighters and sophisticated social networking, the Islamic State already had a global reach. Its acquisition of resource-rich territory made it the wealthiest terrorist organization in history.

Since then, the Islamic State has demonstrated the capacity to adapt and innovate, combining the most effective terrorist practices honed over the last three decades. Nonetheless, beyond capability, what really makes the Islamic State scary is intent.

To sustain its narrative as a true caliphate, the Islamic State must live up to its image as a powerful and growing force. And the most effective way to do that is to take the battle to its enemies via transnational terrorism.

It is folly for the administration to claim that these twin dangers — regional and transnational — are contained. They are unfolding across Europe and most dramatically in Paris as America watches.

Clearly, our strategy for dealing with the Islamic State — such as it is — has been insufficient. The Islamic State has been resilient, absorbing months of drone attacks and lethargic bombing.

Russia’s entry into the war has helped prop up Assad and perpetuate further conflict, but has not driven the Islamic State from the field. While the terrorist group has seen some tactical reversals, like the recent Kurdish assault on Sinjar, it has suffered tactical setbacks before and bounced back.

At this point, the U.S. shouldn’t try to solve Syria. As long as Moscow and Tehran back Assad, he will stick around — as will the war against him. Moreover, it’s not reasonable for the U.S. to lead in the fight to get the Islamic State everywhere.

But where the U.S. could lead is in breaking the Islamic State’s territorial control of Iraq — and that’s a task worth doing because commanding a state is what makes the group a global threat.

Without U.S. participation, the prospects for a successful conventional campaign against ISIS are slim.

Arab states lack both the capacity and expertise to undertake these operations. The Kurds can defend themselves and even regain territory, but liberating the country is beyond them. The Iraqi military is far from ready. Neither Sunnis nor Shia are ready to risk an all-out war with ISIS. European countries don’t have the means to carry out major out-of-area operations on the ground without the U.S. standing by their side.

Additionally, after conventional forces drive ISIS from the field, they will have to keep them from coming back until Iraq is politically stable enough to stand up to external security threats.

This is unwelcome news for a president who boasted of ending the war in Iraq and withdrawing U.S. troops. But the absence of U.S. boots on the ground changed the facts on the ground as well, and the Islamic State was quick to take advantage.

Adding force incrementally — 50 special ops troops here, a few more airstrikes there — cannot substantially alter the current unpleasant facts on the ground. Delay of meaningful ground action risks allowing the twin dangers of the Islamic State to spread even farther.

A 25-year Army veteran, James Jay Carafano is vice president of defense and foreign policy studies for The Heritage Foundation (heritage.org), a conservative think-tank on Capitol Hill.