Two U.S. Navy F-18E Super Hornets support operations against the Islamic State. (Staff Sgt. Shawn Nickel/U.S. Air Force via Agence France-Presse)

Each week, In Theory takes on a big idea in the news and explores it from a range of perspectives. This week we’re talking about just war theory. Need a primer? Catch up here.

Aaron MacLean is a combat Marine veteran of Afghanistan and the managing editor of the Washington Free Beacon.

President Obama announced in September 2014 that he would take military action against the Islamic State to “degrade and ultimately destroy” the organization. It soon became clear that “ultimately” was the operative word in his remarks.

A year later, and the Islamist proto-state has lost territory in some of its core areas, gained territory elsewhere and retained its grip on Mosul in Iraq and its capital of Raqqa, Syria. In recent months, it has launched an international terror campaign and made significant progress expanding operations in Afghanistan and especially in Libya.

To say that the situation is a “stalemate” is a generous assessment, and what is worse, the stalemate is self-imposed. The United States has placed major restrictions on its own conduct of a fight that it nonetheless deems to be necessary, thus prolonging the violence, putting American troops at risk in operations that achieve indecisive results and contributing to global instability. The way we are waging this war is immoral.

[Other perspectives: There are no more just wars.]

As a young Marine officer, I was taught that achieving decisive results in warfare is critical. It even rises to the level of a moral imperative, because — as the Corps’ tactics manual still puts the matter — “An indecisive battle wastes the lives of those who fight and die in it. It wastes the efforts of those who survive it as well. All the costs … are suffered for little gain.” But the counter-Islamic State campaign generates little besides indecisive clashes.

The administration defends the slow pace of progress by insisting that the ground fight against the Islamic State must be waged by local forces. The U.S. military has also imposed strict rules of engagement to limit civilian casualties and minimize environmental damage. These are defensible goals, but ones that have slowed the campaign’s progress to a degree that outweighs their good intentions.

Consider the situation of those living under the caliphate’s rule who do not share its political and religious vision or, worse, who are not Sunni Muslims. The regular reports of mass murder, rape and slavery tell us that the Islamic State’s very existence constitutes a humanitarian catastrophe. It is clear how striving to prevent civilian (and, indeed, coalition military) casualties is a moral act, but if that effort substantially prolongs the existence of a network making life hell for millions — not to mention killing many thousands and generating an enormous refugee crisis — the moral calculus becomes less clear.

Though Iraqis and Syrians are bearing the brunt of this slow-burning conflict’s costs, the humanitarian argument is not the only way to justify calling our conduct in this war immoral. Every day, dozens of air crews take to the sky to strike Islamic State targets, putting their own lives at risk in military actions that by now are quite clearly failing to achieve decisive consequences on the battlefield. It is difficult, but sometimes necessary, for a nation to ask its servicemen and women to risk their lives in combat in order to advance their country’s policy goals. To ask them to wager everything when we know their actions are achieving very little is wrong.

[Syria is a modern-day holocaust. We must act.]

Moreover, as the Paris attacks and recent terror strikes in Tunis, Beirut and Sinai show, the continued existence of the Islamic State endangers the citizens of every nation. The world can only poke ineffectively at an organization like this for so long before it punches back. Whether these attacks were mounted due to a desire for revenge, a wish to bring about an apocalyptic final battle or to demonstrate credibility despite minor losses suffered is immaterial. They would not have happened at all if the Islamic State no longer existed.

The problem of who would “hold” terrain in Syria following the defeat of the Islamic State is a real one, as is the fact that the organization, relieved of its territory, would initially transition to a more traditional terrorist group still capable of exacting harm. Yet the continued existence of a self-declared caliphate is a dangerous affront to an international order that stands for stability and peace, and which promotes human dignity and economic flourishing.

Prolonging the fight against the Islamic State when we have the power to bring it to a decisive conclusion — by means of a less restricted, better-resourced air campaign, supported by American special operations troops fighting alongside local forces, and the limited use of conventional troops at critical moments — merely prolongs suffering and worsens instability.

Marine Corps doctrine also taught me that decisive engagements must lead to beneficial end states. Here, the aftermath will not be perfect, but it will be superior to the present situation, with the Iraqi government again in control of its territory, less-toxic Sunni groups dominant in eastern Syria and Assad no longer able to use the Islamic State as leverage against calls for his ouster. American troops should remain in modest numbers in Iraq, as they ought to have in 2011, when the regional situation was still largely peaceful.

The leaders of the Islamic State invite a final battle with western powers in Syria because, as apocalyptic madmen, they believe the showdown will advance their goals. The only moral thing is to indulge them — with enough resources and a strategy to finish the job.

Explore these other perspectives:

Jeff McMahan: Syria is a modern-day holocaust. We must act.

Marc Pierini: For Europe, war against the Islamic State is justified. But is it worth it?

Thomas Donnelly: The fight against the Islamic State is no ordinary war

Neta C. Crawford: Is the Islamic State a government or a criminal gang? The answer will determine how we fight.

Thomas Madden: The Islamic State’s members believe they are fighting a new Crusade. They’re wrong.

Nolen Gertz: There are no more just wars

Andrew Tabler: Sometimes the enemy of your enemy is still the enemy