Defense Secretary Jim Mattis listens as U.S. President Donald Trump answers questions during a meeting with military leaders in the Cabinet Room on October 23, 2018 in Washington, DC. Win McNamee | Getty Images

U.S. President Donald Trump's abrupt announcement this week that he intends to withdraw all American troops out of Syria risks dealing a serious blow to his country's credibility as an ally and partner, former national security officials and regional experts warned. That decision, announced in a Twitter post, was reportedly the "breaking point" for Secretary of Defense James Mattis, who submitted his resignation letter a day later. The 68-year-old retired Marine Corps general said he was leaving the administration in part because he does not agree with Trump on a number of issues, and cited the importance of alliances. Geopolitical experts are also sounding the alarm on the state of America's international partnerships. "(Trump's Syria move) risks not only jeopardizing the near-term U.S. interest of stabilizing a key part of the Middle East, but also damaging America's reputation for the long term," Turkey expert Soner Cagaptay and former Defense Department and Senate Foreign Relations Committee member Dana Stroul wrote in a brief for The Washington Institute for Near East Policy. Trump has long opposed U.S. military involvement in Syria, and his backers view the withdrawal decision as a campaign promise kept. He announced the defeat the Islamic State (IS), arguing that America should no longer fight others' battles for them. But defense officials and lawmakers reject the assertion that IS is finished, and say that America still has commitments to allies on the ground and a reputation to uphold. "Next time the U.S. needs to challenge an imminent terror threat somewhere in the world, we'll presumably want to do so 'by, with & through,' using local partners," wrote Charles Lister, senior fellow at the Middle East Institute and author of "The Syrian Jihad." "You think they're going to trust us now? Not a chance."

The Kurdish alliance

That sentiment was keenly felt in Northern Syria, where America's local partners in the fight against IS have expressed concern they're being abandoned. The U.S. drew up the Syrian Democratic Forces (SDF), comprised of Arab and Kurdish fighters, as a local partner on the ground after IS swept half of Syria in 2014. Military officials describe the Kurdish element of that coalition — known as the Kurdish People's Protection Unit, or the YPG — as by far the most effective fighting force within the group. The Kurdish fighters receive U.S. training, weapons and air support, but some members of the group have been accused of human rights violations, including ethnic cleansing and the torture of Kurdish, Arab and Turkmen communities. Counter terrorism partnerships in the Middle East have long been messy and complex affairs for Washington.

Members of the Syrian Democratic Forces (SDF), trained by the US-led coalition, participate in the graduation ceremony of their first regiment in al-Kasrah, in the suburb of eastern Syrian city of Deir Ezzor, on May 21, 2018. DELIL SOULEIMAN | AFP | Getty Images

That partnership has long drawn fierce opposition from coalition ally Turkey, which sees the Kurdish militia as tied to the Kurdistan Workers' Party, a U.S.-designated terrorist group that's waged a decades-long insurgency against the Turkish state. Turkey has publicly threatened an imminent attack on Kurdish-held territory in northern Syria, and Trump reportedly told the country's leader that Ankara could do as it pleases. The Kurds now warn a fight with Turkey will detract from their ability to contain the remaining IS forces. "A Turkish incursion would force many YPG fighters to shift their efforts away from fighting IS, risking a reversal of recent progress," Stroul and Cagaptay wrote. "The war against terrorism has not ended and (the Islamic State group) has not been defeated," an SDF representative said in a statement. "The decision to pull out under these circumstances will lead to a state of instability and create a political and military void in the region and leave its people between the claws of enemy forces." Multiple media reports quoted Kurdish leaders and activists labeling Trump's move a "betrayal," pointing to the thousands of Kurdish fighters killed in the anti-IS fight. Tweet1 The shift is catching attention throughout the Middle East. "This sudden change in policy is worrying ... to all U.S. allies in the region," former Iraqi foreign minister Hoshyar Zebari told the Washington Post. "It's a question of trust. This will cause many governments to rethink their alliances with a superpower that can throw them under the bus." The shock has rippled to some American forces, where reports from NBC describe "U.S. special forces troops distraught, upset, morally disturbed by having to tell their Kurdish allies in Syria that, because of orders, their promises of defense won't be kept." The Pentagon, White House and State Department did not respond to CNBC requests for comment. In a Saturday night Twitter post, Trump said allies "are very important-but not when they take advantage of U.S." Donald J. Trump tweet: When President Obama ingloriously fired Jim Mattis, I gave him a second chance. Some thought I shouldn't, I thought I should. Interesting relationship-but I also gave all of the resources that he never really had. Allies are very important-but not when they take advantage of U.S.

'Americans have to be realistic'