President Trump contends that his withdrawal of U.S. military forces from Syria will serve American interests without compromising the critical interests of close allies. One Syrian town is about to test that thesis.

Namely, the situation in the northern-central city of Manbij. If Trump fails to exert U.S. leverage in relation to Manbij, that city will become a marker for his version of former President Barack Obama's red-line chemical weapons debacle in Syria. Located just west of the Euphrates river and about 50 miles northeast of Aleppo, Manbij is currently controlled by U.S.-allied Kurdish forces. But now that Trump has ordered a withdrawal of U.S. forces from Syria, other forces are moving in on Manbij. These elements know that controlling the city means controlling the access routes to Aleppo, to Turkey, and across the Euphrates to Ain Issa and the Kurdish strongholds farther east. In short, Manbij is a strategic gemstone.

Yet the various warring parties in Syria have different views over who should control Manbij. Syrian President Bashar Assad wants to control Manbij in order to dominate Syria's northwest. Turkish President Recep Tayyip Erdogan wants to control Manbij as a buffer against the Kurds and an influencing point against Assad. The Iranians support Assad. The Sunni-Arab kingdoms also support Assad as a means to protect their own influence (more on that in another piece later). And the Russians? They love the chaotic jockeying in that it puts Russian President Vladimir Putin in his favorite position: as supreme dealmaker in chief. Russia's strategic interest is in fostering chaos so as to demand favors for exerting its leverage on the ground.

This is a major challenge for Trump. If the president allows the Kurds to be smashed by Turkey, or dominated by Assad, or made feudal serfs to Russia, U.S. credibility with its Middle Eastern partners will evaporate. Note, here, that perception of power matters as much as practical power in Middle Eastern politics. Thanks to his abrupt Syrian withdrawal, at the moment, Trump is perceived as unreliable. This is a big problem for the president's broader objectives in the Middle East: consolidating a Sunni-led counterterrorism alliance, strengthening balance of power relationships with Sunni kingdoms, and consolidating regional politics away from Iran.

If Trump is seen to abandon allies at critical moments such as that of Manbij, regional leaders will turn to those they believe are more useful in America's vacuum. And that's Russia. Seeing as Russia's overarching game plan in the Middle East is to play various actors off against each other, Russia's rise will mean only one thing: more chaos, the exact ingredient for conflict, instability, and terrorism.

Trump can act here. He can demand to Erdogan that the Turks not enter Manbij. He can also continue to provide U.S. logistical support to the Kurds and cut a deal with Assad that balances the interests of American allies. But Trump's present course suggests he believes no interests are at stake in Syria. He is wrong. In physical and credibility terms, U.S. interests are on the line.

When it comes to defending those interests, the buck stops at the Oval Office desk.