The controversy over Trump's plan to pull US troops out of Syria, explained

The White House made an announcement on Sunday that shocked its allies in Syria. President Donald Trump's administration announced that Turkish forces would soon move into portions of northern Syria which are currently controlled by Kurdish groups, and that US troops in the area would step aside.

While the Kurds have been Washington's staunchest and most effective allies in the war against ISIS, it now appears they will be left to Turkey's mercy.

Turkey's military has begun to cross into northern Syria, the Turkish government said early Wednesday, as part of an impending offensive to move Kurdish forces away from its border.

It comes after the Trump administration released a statement saying US forces "will not support or be involved in the (planned Turkish) operation" and that American troops "will no longer be in the immediate area."

As of last month, the US said about 1,000 US troops were operating in northeastern Syria. Sunday's statement did not specify if this constituted a full withdrawal of personnel from the country.

But what's behind Turkey's move to enter northern Syria? What is the potential impact of the US pull-out? And how will it impact the region's security? Here are some answers:

The devastating war in Syria has recently moved into a new phase which could be described as one of "consolidation." With the help of Russia and Iran, the Syrian military has managed to all but defeat the insurgency looking to unseat Syrian President Bashar al-Assad.

At the same time, the US-led coalition with the Kurdish-dominated SDF -- and to an extent Russian and Iranian forces -- managed to drive ISIS to the brink of destruction, at least territorially. ISIS holds virtually no more territory but the US warns there are still tens of thousands of ISIS fighters in hiding in both Iraq and Syria.

The SDF also still holds thousands of ISIS fighters captured during major battles to unseat the terror group. Many of those fighters are from foreign countries, often from Europe. The Trump administration has long been warning European nations that the US will have to release them if the Europeans don't take their citizens back. If Turkey moves into northern Syria, it would probably take control of most of the prisons where ISIS fighters are currently being held.

The Kurdish-led Syrian Democratic Forces -- the Kurdish group backed, until now, by the US in Syria -- reacted with anger and disgust, saying the US pull-back would jeopardize regional security but vowing to defend the territory they fought so hard to liberate from ISIS.

Shervan Darwish, a spokesman for Syria's Manbij Military Council -- which is a coalition of SDF groups -- said it "will give a morale boost to ISIS's sleeper sells and will create a vacuum that ISIS will certainly use."

"This Turkish military operation in northern and eastern Syria will have a large, negative impact on our war against ISIS and will destroy all the stability that has been accomplished over the past years," another SDF statement on Monday said.

Brett McGurk, the former US envoy to the anti-ISIS coalition, also heavily ripped into the Trump administration's plans.

"The WH statement tonight on Syria after Trump spoke with Erdogan demonstrates a complete lack of understanding of anything happening on the ground. The 'United States' is not holding any ISIS detainees. They are all being held by the SDF, which Trump just served up to Turkey," McGurk tweeted Monday.

After ISIS took over large parts of northeastern Syria and northwestern Iraq years ago, the US put together a coalition to combat the terror group. While Western powers brought a lot of air power to the battlefield, they always had problems mustering a viable ground force to reclaim land. The Iraqi and Syrian Kurds proved to be among the most effective anti-ISIS fighters, and Washington relied on them to not only conduct ground offensives but even to call in US airstrikes.

As part of the effort to make the mostly Kurdish SDF more effective, the US and other nations embedded special forces with them. At the end of most of the Syrian combat operations, Turkey began voicing concern about the strong Kurdish presence in northern Syria. Ankara has been in conflict with Kurdish insurgents, both inside and outside Turkish borders, for decades.

The US left forces in Syria to continue the fight against ISIS, but also to prevent Turkey from invading northern Syria and driving out the US-supported Kurdish militias there.

But now, the withdrawal of US troops will open the door for Turkey to push back America's former allies.

The Turks have long been extremely unhappy about the strong Kurdish presence in northeast Syria near the Turkish border. Turkey's military has already moved into portions of areas previously held by SDF, but now it seems that Ankara has the green light from Washington to follow through on its plans to create a buffer zone in northern Syria.

Turkey does not only want to ensure that the Kurdish forces withdraw from these areas, they also want to resettle around 2 million Syrian refugees there. During the worst of the fighting of the Syrian conflict, around 3.6 million Syrians fled to Turkey. Many of those people are now either living in giant makeshift camps or in towns in the border region. The Turks hope to achieve two objectives by creating a buffer zone: drive the Kurds away from their border and repatriate a large number of Syrians.

The US military has long warned that the fight against ISIS is not over. While the terror group has lost the territory it used to call its "caliphate," there are still tens of thousands of ISIS fighters on the loose in both Syria and Iraq, according to Washington.

On top of that, tens of thousands of people formerly living in ISIS-controlled areas in northeastern Syria -- some of them staunch supporters of the group -- have been taken to large camps. The largest is al-Hol, which has around 50,000 inhabitants.

Kurdish-led groups also hold thousands of ISIS prisoners, while ISIS leader Abu Bakr al-Bagdadi has recently called on his supporters to break the ISIS prisoners out of jail.

So while ISIS is weakened, it is still around, and still dangerous. This was the main reason the US gave for staying in Syria, even after ISIS lost its last enclaves earlier this year. ISIS, the US believes, still has the power and capability to regroup and re-emerge. The Kurds have warned that the chance of ISIS coming back will rise considerably if Turkey moves into northeast Syria.

"We were doing our best to provide the best kind of security in the prisons and in the camps ... [but] with the Turkish invasion ... we are forced to pull out some of our troops from the prisons and from the camps to the border to protect our people," SDF spokesman Mustafa Bali told CNN.

While the Kurds have vowed to defend the territory they currently control, it is hard to see them matching up against the well-equipped and powerful Turkish military. The only way out for the Kurds may be seeking protection from Russia and its ally, Syrian President Bashar al-Assad.

While Syria's Kurds have long wanted more autonomy from Damascus, they are not against the Assad government per se. In fact, Kurdish fighters fought alongside the Syrian military in some of the battles of the country's civil war.

Faced with a Turkish incursion, the Kurds might already be on the phone to Assad and the Russian military since they certainly would view Syrian government control as a lot better than a battle against the Turkish army.

Another big question is what will happen with the many displaced people in the former ISIS-controlled areas and the ISIS fighters now being held by the SDF. Former US envoy McGurk believes Turkey is ill-equipped to deal with the situation, while President Trump has made it clear that he does not want to deal with it any longer. It's very difficult to predict what will happen next in northern Syria, but the immediate future after a US pull-out certainly does not look bright.