The American people have been told consistently by military and political leaders that U.S. objectives in Syria are straightforward. The establishment says U.S. action against the Islamic State is essential to protecting national security; and when ISIS’s self-proclaimed caliphate is destroyed, the entire Middle East will be more stable.

Speaking at a Brookings Institution event last month, Chairman of the Joint Chiefs of Staff Joseph Dunford told the audience the Trump administration’s strategy will continue to follow the paradigm of advising, assisting, and training Iraqi troops and Syrian militia units before and during their operations.

So why are U.S. troops now effectively acting as peacekeepers in the northern Syrian city of Manbij? It’s a mission that by every indication is a dramatic expansion of the original U.S. strategy of training and advising anti-ISIS ground units.

According to Pentagon spokesman Navy Capt. Jeff Davis, a small detachment of U.S. soldiers were deployed to Manbij to keep an ethnic tinderbox from exploding. Due in large part their own four-decade-long counterinsurgency against Kurdish militants in southeastern Turkey, Ankara has looked upon Kurdish expansionism in Syria over the last two years as a morale booster for the PKK, a Turkish terrorist group that remains committed to Kurdish autonomy. The Syrian Kurds, in turn, tend to view the Turkish government as a persecutor of Kurdish aspirations in the region. As Davis summarized to reporters last week, “[t]his is obviously a really complicated situation.”

It is not an exaggeration to call the battlefield dynamics in Manbij “really complicated.” The city was held by ISIS for years until this summer, when it was liberated by the Arab-Kurdish militia collectively referred to as the Syrian Democratic Forces (SDF). Ideally this would be the end of the story, but in Syria’s complex, multi-sided, proxy civil war, nothing is simple.

Turkey has threatened to move in and forcefully expel the Kurds. Manbij has become a heavily congested area with multiple factions with multiple interests only a stone’s throw away from each other. SDF troops, local Arab fighters, pro-Syrian government forces, and Turkish soldiers all view the city as critical geostrategic terrain.  

And in the midst of it all, Washington—without a debate in Congress—ordered a few dozen U.S. special forces to keep the peace. This detachment is charged with ensuring that Turkish soldiers and what is left of the Kurdish element in the city don’t complicate the larger war by starting yet another ethnic conflict.

The Pentagon casually conceded that “this is a different mission” from the immediate focus on the Islamic State, as if sending U.S. soldiers into a situation as quasi-peace enforcers was an organic and logical extension of a counterterrorism campaign.

The Pentagon’s logic is simple: The U.S. cannot afford to have our allies in the fight get distracted by shooting at each other, so we need U.S. troops to ensure these factions remain focused on fighting ISIS instead of each other.

Just because the logic is simple, however, doesn’t mean the mission itself shouldn’t be questioned. It’s difficult to look at the Manbij operation and conclude anything other than that this is yet another instance of “mission creep.”

Asking U.S. soldiers to keep Turks, Syrians, and Kurds in their own lanes in order to avoid an armed escalation is not even tangentially connected to the military campaign against ISIS, despite the Pentagon’s attempt to explain the shift in objectives.

Keeping the Kurds and Turks from killing each other in a border city is not even remotely similar to the counterterrorism mission the U.S. military has led in Syria and Iraq for the last three years. And policymakers who believe this is a good idea haven’t sufficiently explained how this new mission isn’t a distraction and an expansion.

Reasonable people can disagree about how close U.S. special operations forces and advisers should be to the front lines in Mosul—or whether it’s a wise course of action to pour more U.S. military resources into Syria to speed up the liberation of Raqqa. These are debates over tactics rather than strategy: killing ISIS militants, targeting their leadership and funding sources, isolating their transportation and re-supply routes, and driving them from urban areas are all in service of the overall goal of wiping ISIS off the battlefield.

But using the U.S. military to deter an ethnic skirmish between Turkish soldiers, Turkish-supported Syrian proxy forces, and Kurdish militias—at considerable risk to American personnel—is a far different mission.

Three years ago, you would have been hard pressed to find an official in the national security bureaucracy who would have predicted that the U.S. military would be playing referee between two antagonists that have intense, multi-decade grievances against one another, all in a city that has been free of Islamic State presence for over six months. It is equally unlikely that you would have discovered anyone in the bureaucracy who would support converting U.S. soldiers into a security buffer with Turks on one side and Kurds on the other—a deployment that would serve no purpose other than temporarily cooling one of the region’s most intractable ethnic conflicts.

The Pentagon may consider a de-facto peacekeeping mission in Manbij as part of the counter-ISIS campaign, a way to ensure that Washington’s allies in Syria devote their energy to fighting the one enemy that matters. But for those of us who are constantly on alert to anything that looks like a disruption to our core mission, the Manbij operation is mission creep, plain and simple.  

Manbij is an unnecessary sideshow to the central counterterrorism objective that America’s men and women in uniform have been ordered to perform: destroying whatever is left of ISIS’s self-described caliphate. If Manbij-style operations become the norm, the foreign-policy establishment in Washington will once again have exposed itself to the American people as either too arrogant or too ignorant to learn the lessons of the past sixteen years. The U.S. military are the guardians of U.S. security, not the world’s rapid-response police force.

Daniel R. DePetris is a fellow at Defense Priorities.