Top U.S. defense officials believe all of the warring parties in Manbij got the message after American forces deployed an armored convoy through the Syrian city earlier this week. But while the U.S.-led coalition may have temporarily prevented Russians, Syrians, Turks and Kurds from attacking and killing each other, the situation there exposes far graver dangers for future conflict in Syria.

It was only a matter of time before the Syrian regime's civil war in the western part of the country, backed on the ground and in the air by Moscow, abutted the U.S.-led coalition's fight against the Islamic State group in the east. Those forces' convergence in Manbij, a key transit hub toward the terrorists' capital in Raqqa, was troublesome enough on its own, but it comes at a time when all fighting forces in Syria see an opportunity to establish strongholds today that might grant them greater regional control in the future.

The showdown in Manbij this week exposes that President Donald Trump needs to make a decision soon about which forces the U.S. will support in the final campaign to defeat the terrorist network and how the White House plans to follow through without infuriating other key allies. It's a nearly impossible choice, and the administration needs to decide fast.

"This is one of the most nuanced and complex conflicts I've ever seen in my life," says Julianne Smith, a former defense official and national security adviser to Vice President Joe Biden, now with the Center for a New American Security. "It's not black and white. There isn't good guys and bad guys, in fact it's anything but."

"It's a signal to the [Trump] administration: Time is short. Pick a side. But the problem is picking a side is the hardest part in all of this."

Indeed it is. The Kurdish militia known as the PYG has been among the most successful in retaking ground from the Islamic State group and has settled into Manbij after clearing it. But Turkish officials fear that the Kurds are really establishing a potential stronghold that could bolster their efforts to create a breakaway region once the fighting against the Islamic State group stops.

Ankara is concerned that groups like the PYG are connected to terrorist organizations at home and their aspirations for independence could destabilize the NATO ally. Turkey, a crucial tactical partner within the U.S.-led coalition, launched a new campaign into northern Syria last year that only partially coordinated with the coalition leadership and was aimed at breaking up Kurdish positions. It's unclear when, if ever, Turkey plans to withdraw.

Further complicating the situation in Manbij was the arrival from the west of Syrian troops accompanying a government aid convoy that was also protected by an element of Russian ground forces. The Manbij Military Council – an Arab group operating as the de facto government there – denied reports this week that it had handed over portions of the city to regime forces for them to control.

"This is one of the places where you could imagine front lines could remain for a while," Michael O'Hanlon, a senior fellow at the Brookings Institution, says of Manbij, adding that a protracted standoff could also easily descend into a crisis if one the warring groups assembled there were to lash out at the other.

Each of these elements puts the U.S. in a precarious situation: Angering the Turks by continuing to rely on Kurdish militia risks Ankara's withdrawing permission for U.S. bombers and fighters to use its airfields, most notably Incirlik air base. American defense officials say privately that losing access to those bases would significantly hamper the U.S. air war. The coalition may have no other choice than to rely on the Kurds, however, as the Arab forces on the ground, known as the Syrian Democratic Forces, do not have the necessary equipment, training or sheer numbers to retake Raqqa, which Pentagon experts expect will be a hard and bloody battle.

And throughout these conflicts is the omnipresent concern that Russian forces and their Syrian proxies could engage the U.S. military presence and the ground forces it's compiled – a potential escalation narrowly avoided in recent months following Russian warplanes striking U.S.-trained rebels.

On Monday, U.S. commanders directed forces already based in Manbij to drive a series of convoys through the city and surrounding area with American flags flying. It was designed to send a clear message: Everyone needs to continue the momentum against the Islamic State group, not each other.

Army Gen. Joseph Votel, who oversees U.S. operations in the region, says the city is now stabilizing.

"Part of our intention is to operate in a manner that makes it very clear that the coalition forces are there, and our focus is on ISIS," the general said after testifying before a congressional panel in Washington, D.C., on Thursday. "That's the whole idea here: Keep people focused on the mission at hand."

The situation comes as the White House considers a plan the Pentagon submitted in February for changes to the campaign against the Islamic State group. Defense Secretary Jim Mattis has already instituted some slight changes, including the deployment of 300 Marines to Syria to provide artillery support for a ground force once the siege on Raqqa begins.

Straining U.S. efforts, and concerning many Middle East experts, is how few positions the Trump White House has been able to fill at the State Department and Pentagon, where the respective secretaries and their closest advisers remain the only political appointees currently working these problems.