"To be allies is to fight shoulder to shoulder. It's the most important thing for a head of state and head of the military. An ally should be dependable," he said, according to Reuters.

"I very deeply regret the decision made on Syria," Macron said while at a press conference in Chad on Sunday.

Trump first made the announcement via Twitter last Wednesday in a move that blindsided lawmakers and officials and drew strong opposition from Republicans and Democrats alike.

Macron was referring to the local forces that Washington has supported since it first entered Syria's nearly eight-year long civil war in 2015. Roughly 2,000 U.S. troops, including special forces, work in partnership with the Syrian Democratic Forces (SDF), an amalgamation of Arab and Kurdish militias that do most of the fighting against the so-called Islamic State (IS) on the ground while the U.S. provides training, weaponry and air support.

Of the SDF, the Syrian Kurdish militia fighters of the Kurdish People's Protection Units (YPG) have been described by the Pentagon as the most effective partner in driving IS out of the region, and have lost thousands of fighters since the counter-IS offensive began in 2014.

Now, those forces will be left on their own to combat what remains of IS — and national security experts warn that without the U.S. presence, the Islamic extremist group will see a resurgence.

What's more, Turkey's President Recep Tayyip Erdogan is threatening an imminent invasion of Kurdish-held territories in northern Syria. Ankara sees the YPG as intimately tied to the Kurdistan Workers' Party, a designated terrorist group that's led a decades-long insurgency against the Turkish state. Erdogan has amassed Turkish troops along the Turkish-Syrian border and is threatening to "cleanse" the area of "terrorists," a move the Pentagon has repeatedly called unacceptable.

Experts now fear that a Turkish offensive will force the Kurdish militias to abandon their counter-IS efforts in order to focus on defending themselves from the Turks. YPG forces are reportedly already digging ditches and trenches in preparation for a Turkish onslaught. On hearing those reports, Turkish Defense Minister Hulusi Akar said Thursday, "they will be buried in their ditches when the time comes."

Trump, defending his decision via Twitter, said on Monday: "President @RT_Erdogan of Turkey has very strongly informed me that he will eradicate whatever is left of ISIS in Syria....and he is a man who can do it plus, Turkey is right "next door.'"

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The president wrote earlier that "allies are very important — but not when they take advantage of U.S."

Trump's backers, meanwhile, see his move as a campaign promise kept. Many others support the decision as a means of ending a Middle Eastern military engagement they say had no endgame and no defined outcome.

"Americans have to be realistic. The problems of ISIS and the Syrian Kurds are not going be settled by 2,000 American special forces," former U.S. Ambassador to Syria Robert Ford told NPR on Friday. "ISIS is smaller, it can be managed by Syrians, and it should be managed by Syrians."