Donald Trump has doubled down on his unexpected decision to pull U.S. troops out of northern Syria despite backlash from critics on all sides, who argue he is leaving Kurdish allies open to an attack from Turkish forces.

The announcement and criticism come as the president is facing an impeachment inquiry into his dealings with Ukraine's leader.

Here's what we know about the announcement that has even Trump's closest allies in shock:

Trump announces his decision to pull out of northern Syria

On Sunday, the White House said that the U.S. would withdraw troops from a region of northern Syria that borders Turkey. This comes ahead of a Turkish military incursion into Syria against those it considers terrorist threats.

Among those perceived threats are Kurdish fighters, part of the Syrian Democratic Forces, who have been allied with the United States in fighting the Islamic State, or ISIS.

More:'A stain on America's honor': Lindsey Graham says Trump's Syria pullout abandons Kurds, helps ISIS

Trump defended the move as an attempt to fulfill a campaign promise to end U.S. involvement in wars such as in Syria.

"I campaigned on the fact that I was going to bring our soldiers home," Trump told reporters of the decision.

The White House said the announcement came after Trump talked to Turkish President Recep Tayyip Erdogan, who believes that Kurdish forces are allied with insurgents inside Turkey, on the phone.

U.S. relationships with Kurdish and Turkish forces

The United States has fought alongside the Syrian Democratic Forces against the Islamic State, capturing thousands of terrorists in the region.

"We may be in the process of leaving Syria, but in no way have we Abandoned the Kurds, who are special people and wonderful fighters," Trump said Tuesday.

A State Department official said that Trump’s decision to withdraw two military units from the border was based on two goals: He does not want U.S. forces to be seen as greenlighting Turkey’s plans, and he does he not want Americans in the line of fire if Erdogan goes through with his threat.

The Kurdish fighters said in a statement that their U.S. allies "did not fulfill their obligations."

Trump first announced his intention to withdraw U.S. troops from Syria in 2018. Amid a backlash, he later agreed to keep a residual American presence there. Trump's announced withdrawal from Syria in late 2018 also led to the resignation of then-Defense Secretary Jim Mattis.

Foreign policy experts have warned that allowing Turkey into the region could lead to a massacre of the Kurds and would be seen as an American betrayal of a vital military ally.

Trump tweeted on Monday, "WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN."

Who are the Kurds?:A Middle Eastern people with 'no friends but the mountains'

What is Turkey expected to do next?

According to the White House, the move from the United States does not signal support of the Turkish mission to invade Syria.

"Turkey will soon be moving forward with its long-planned operation into Northern Syria. The United States Armed Forces will not support or be involved in the operation," the White House said.

Turkey has long expressed its intentions to quell perceived threats in the region.

The White House says that captured ISIS fighters will now be under Turkey's control. If they're released, they could return to the battlefield.

Trump said that Turkey "must, with Europe and others" guard over the captured Islamic State fighters held by the Syrian Democratic Forces if Erdogan sends his forces into Syria.

The SDF warned that Turkey's planned "invasion" would "have a major negative impact on our war on ISIS and will destroy all the stability achieved during the past years."

The Kurdish fighters said they "will not hesitate for a moment to defend ourselves" and they called on "Arabs Kurds, Syrians and Assyrians to join forces and stand with their legitimate forces to defend our country against this Turkish aggression."

A spokeswoman for the European Union warned that Turkey's planned military action "will only exacerbate civilian suffering and lead to massive displacement."

"If Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I’ve done before!)," Trump tweeted.

Erdogan has also reportedly accepted an invitation from Trump to meet in Washington.

The bipartisan argument against withdrawal

Lawmakers on both sides of the aisle have warned that this withdrawal could lead to a massacre of Kurdish people.

Republicans like Sen. Lindsey Graham of South Carolina, and Senate Majority Leader Mitch McConnell of Kentucky, who are normally staunch Trump supporters, expressed disapproval of the president's actions in statements and interviews.

"A precipitous withdrawal of U.S. forces from Syria would only benefit Russia, Iran, and the Assad regime," McConnell said. "And it would increase the risk that ISIS and other terrorist groups regroup."

Graham said that the decision is "a disaster in the making."

"I like President Trump," Graham said on Fox and Friends. "I've tried to help him. This to me is just unnerving to its core."

'A reckless gamble':Reasons critics decry Trump's 'impulsive' Syria withdrawal

Nikki Haley, who was Trump's ambassador to the United Nations, tweeted that "The Kurds were instrumental in our successful fight against ISIS in Syria. Leaving them to die is a big mistake."

Graham said he and Sen. Chris Van Hollen, D-Md., planned to introduce bipartisan sanctions on Turkey if they invade Syria and to call for Turkey's suspension from NATO if they attack the Kurds.

Democrats, too, weighed in on what they've called a "sinister" move.

"The U.S. convinced the Kurds to destroy ISIS for us, causing massive Kurd casualties. Then we conned the Kurds into dismantling their defenses, promising to protect them. Now Trump invites the Turks into Syria, green lighting them to wipe out the Kurds," tweeted Sen. Chris Murphy, D-Conn., a member of the Senate Foreign Relations Committee. "Positively sinister."

"Trump’s narcissism is a threat to our national security and stability around the globe," tweeted Rep. Katherine Clark, D-Mass.