A member of the US forces stands guard during a meeting between US special Presidential Envoy for the Global Coalition to Counter IS and the Tabqa Civil Council in the town of Tabqa, about 55 kilometres (35 miles) west of Raqa city, on June 29, 2017. DELIL SOULEIMAN | AFP | Getty Images

President Donald Trump's abrupt announcement Wednesday to withdraw U.S. troops from Syria has alarmed Trump allies and opponents alike, sparking fears it will backfire on American goals in the region. Critics warn the dramatic policy turnaround will hurt U.S. counterterror operations, diminish its influence on the ground, and bolster freedom of movement for Iran and remaining Islamic State, or ISIS, militants in Syria. In an even more unexpected move, the U.S. will end all its air activities — including strikes against ISIS — as part of the withdrawal, Reuters reported Thursday. "Withdrawal of this small American force in Syria would be a huge Obama-like mistake," Sen. Lindsey Graham, R-S.C., an otherwise staunch Trump ally, tweeted Wednesday. "A decision to withdraw will also be viewed as a boost to ISIS desire to come back." Tweet1 Graham was responding to Trump's initial tweet attempting to justify the Syria shift, which said: "We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency." While a Pentagon spokesperson affirmed the campaign against ISIS would continue, the new plan is to withdraw U.S. troops from Syria within 60 to 100 days. Sen. Marco Rubio, R-Fla., said in a Facebook livestream that "while ISIS has been significantly degraded ... it is not fair or wise to say that they have been defeated. This is a bad idea because it actually, it goes against the fight against ISIS, and potentially helps ISIS."

We should not and cannot allow ISIS to breathe at this critical point or we will jeopardize the significant gains we have made... and risk allowing ISIS to resurge. Navy Cmdr. Sean Robertson Pentagon spokesman

Military officials appear to agree. As recently as Dec. 13, Pentagon spokesman Navy Cmdr. Sean Robertson said: "Coalition forces are working closely with the Syrian Democratic Forces (SDF) who are in the midst of offensive operations against ISIS. ... We should not and cannot allow ISIS to breathe at this critical point or we will jeopardize the significant gains we have made alongside our Coalition partners and risk allowing ISIS to resurge." According to a U.S. military estimate released last summer, up to 30,000 ISIS fighters remain across Syria and Iraq.

A morale boost for Assad and Iran

While critics say Washington didn't have a coherent Syria policy to begin with, the presence of some 2,000 U.S. troops in the country aimed to serve the primary agenda of combating ISIS and containing Iranian influence, while deterring further military action by adversaries in the region. But a U.S. withdrawal should embolden Syrian President Bashar Assad and his Iranian and Russian backers, giving them the freedom to shape Syria's future themselves, experts said. "This means that the U.S. is giving up almost all possibility to have a say in upcoming peace negotiations regarding the Syrian conflict," Agathe Demarais, a principal economist at the Economist Intelligence Unit, told CNBC. "Meanwhile, both Russia and Iran will be keen to capitalize on the U.S. withdrawal from Syria, and to use this as yet another illustration, in their view, of the fact that the Syrian regime won the war and the country is now about to be ready for reconstruction operations led by Russian and Iranian companies." The decision also contradicts the statements of Trump's own top advisors. In September, national security advisor John Bolton said, "We're not going to leave [Syria] as long as Iranian troops are outside Iranian borders and that includes Iranian proxies and militias."

Free Syrian Army fighters fire an anti-aircraft weapon in a rebel-held area of Aleppo, Syria December 12, 2016. Abdalrhman Ismail | Reuters

Amid the chorus of alarm, however, a number of observers point out that there was never an articulated endgame in Syria, and that those opposing a troop pullout may be overestimating what the U.S. could have accomplished if it remained on the ground. The Atlantic Council's Faysal Itani wrote in a policy brief that the withdrawal sacrifices whatever leverage the U.S. had against Assad or Iran, while admitting that that leverage was not particularly huge to begin with. Russia's Foreign Ministry, meanwhile, welcomed the news, slamming America's presence in Syria as "illegal" and a "dangerous obstacle" in the path toward a peace settlement during a Wednesday news briefing. Trump responded to criticism in a tweet Thursday morning. "Does the USA want to be the policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing? Do we want to be there forever?" he wrote. Tweet2

Israeli critics warn of 'all-out conflict'

The concern over Iran was reiterated by former Israeli Defense Minister Avigdor Lieberman, who told Israeli radio on Thursday that a vacuum left by the U.S. would empower Tehran and its proxy force Hezbollah, which has used Syrian territory to carry out cross-border drone and rocket attacks into Israel. In turn, Israel has carried out hundreds of airstrikes targeting Iranian-linked forces in Syria. "The withdrawal of the U.S. from Syria significantly raises the chance of an all-out conflict in the north — both [in] Lebanon and Syria," between Israeli and Iranian-backed forces, Lieberman said. Iranian expansion in the absence of U.S. forces could risk triggering dramatic escalation between the two. Behnam Ben Taleblu, of the hawkish think tank Foundation for Defense of Democracies, sees the move as a "major setback" for Trump's "maximum pressure" policy on Iran. "The removal of U.S. troops from Syria effectively permits Tehran to say they bested the U.S. in a seven-year shadow war in Syria," he said, adding that the haste with which the decision was apparently made echoes the Obama administration's ill-fated decision to rapidly withdraw troops from Iraq in 2011 — a move credited for ultimately allowed one-third of the country to fall to ISIS.