President Donald Trump defended his decision to withdraw US forces from northern Syria — a move which critics say could open the way for ISIS to regroup and for Turkey to attack the US' Kurdish allies.

"The United States was supposed to be in Syria for 30 days, that was many years ago. We stayed and got deeper and deeper into battle with no aim in sight," he tweeted.

The president's decision has taken allies both at home and abroad by complete surprise.

One of Trump's staunchest allies, Sen. Lindsey Graham, said the move would allow ISIS to regroup, which would be "a disaster in the making."

In a series of early Monday morning tweets, President Donald Trump defended his decision to withdraw US forces from northern Syria, which has left allies stunned.

The White House on Sunday said US forces in northeast Syria would move aside and not prevent an expected Turkish assault against Kurdish forces, which had been fighting ISIS alongside US forces.

He appeared to complain of the monetary cost of fighting alongside Kurdish forces, and said that US allies in the region "will now have to figure the situation out." He also claimed that the US had "defeated 100% of the ISIS Caliphate."

Here's what he said:

"The United States was supposed to be in Syria for 30 days, that was many years ago. We stayed and got deeper and deeper into battle with no aim in sight.

"When I arrived in Washington, ISIS was running rampant in the area. We quickly defeated 100% of the ISIS Caliphate, including capturing thousands of ISIS fighters, mostly from Europe. But Europe did not want them back, they said you keep them USA! I said 'NO, we did you a great favor and now you want us to hold them in U.S. prisons at tremendous cost. They are yours for trials.'

"They again said 'No,' thinking, as usual, that the U.S. is always the 'sucker,' on NATO, on Trade, on everything. The Kurds fought with us, but were paid massive amounts of money and equipment to do so. They have been fighting Turkey for decades.

"I held off this fight for almost 3 years, but it is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home. WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN.

"Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out, and what they want to do with the captured ISIS fighters in their "neighborhood." They all hate ISIS, have been enemies for years. We are 7000 miles away and will crush ISIS again if they come anywhere near us!"

The president's decision was made without warning, and has left allies at home and abroad reeling. Even Pentagon officials reportedly received no warning of the decision.

Kurdish forces in northern Syria have fought alongside the US-led coalition to defeat ISIS since 2015. But Turkey considers Kurdish forces to be terrorists, and the US withdrawal paved the way for Turkey to attack them.

The White House said that Trump endorsed Turkey's planned military operation in the region, with Trump having reportedly arrived at the decision after speaking to Turkey's president, Recep Tayyip Erdogan, by phone on Sunday.

Trump suddenly announced in December 2018 that he was withdrawing US forces from Syria, but scaled back the decision following opposition from military, intelligence, and administration officials and allies in Europe and the Middle East.

However Turkey was reportedly unhappy with confidence building measures launched to resolve tensions between Turkey and Kurdish forces, and declared its intention to sweep into the region.

Sen. Lindsey Graham -one of the president's staunchest congressional allies - tweeted that the move would allow terrorist group ISIS to regroup, which would be "a disaster in the making."