WASHINGTON - The furor over the decision to pull U.S. troops out of northeastern Syria began late Sunday night with a White House statement about an ominous telephone conversation between President Donald Trump and the Turkish president.

The results have been rapid and remain unpredictable - and, in the view of critics, amount to the abandonment of America's Syrian Kurdish allies to a massive Turkish military assault.

As Turkish forces hovered on the Syrian border Tuesday, U.S. officials said the attack could come within hours. On Twitter, Trump wrote that the United States had provided arms for the Kurds and warned that any "unforced or unnecessary fighting by Turkey will be devastating to their economy and to their very fragile currency."

Turkey's vice president responded that his country would "not react to threats."

The call between Trump and Turkish President Recep Tayyip Erdogan started out with good news, according to U.S. officials who spoke on the condition of anonymity to discuss the sensitive communication and other developments. Trump summed up several U.S. incentives that had been discussed with Ankara in recent weeks as a way of improving a tattered bilateral relationship. They included a $100 billion trade deal, a solution to Turkey's exclusion from contributions to and purchases of the U.S.-made F-35 fighter jet, and a White House visit by Erdogan.

The two also discussed a U.S.-Turkey agreement, recently implemented, to jointly patrol a "safe zone" in northeastern Syria along the Turkish border, cleared of the Kurdish force to which Ankara objected.

To Washington, the Kurds were indispensable fighters in the war against the Islamic State.

To Turkey, they were terrorists allied with Turkish Kurds seeking autonomy through violence, and the safe-zone agreement was too little, too late. Erdogan told Trump that his troops, massed on the border for months, were ready to move. It's not clear how Trump responded to the warning.

The White House statement, issued about an hour before midnight Sunday, said "Turkey will soon be moving forward with its long-planned operation into Northern Syria." U.S. forces, it said, "will not support or be involved in the operation" and would withdraw from the area.

There was no mention of the Kurds.

The response was brutal. Republican leaders, awakening to news accounts focused on the withdrawal announcement, denounced Trump's abandonment of the Kurds and kowtowing to Turkey. Pentagon officials struggled with explanations, humanitarian workers warned of civilian casualties, and Kurdish commanders said they would abandon the Syrian prisons where they were holding thousands of captured Islamic State fighters and head for the front lines against Turkey.

Trump defended the withdrawal with a dawn tweetstorm, saying "it is time for us to get out of these ridiculous Endless Wars."

National security aides - most of whom had not seen the call announcement the night before - quickly mobilized to repair the damage, and the presidential tone changed.

At 10:38 a.m. on Monday, a new Trump tweet warned that if Turkey did anything Trump didn't like, "I will totally destroy and obliterate the Economy of Turkey."

On Tuesday morning, he switched to tweeted carrots and back to sticks again in the space of 30 minutes, first reminding Turkey of the incentives package, affecting a long list of accumulated Turkish grievances.

U.S. sanctions and tariffs have affected Turkey over a range of issues, including its dealings with Iran and Venezuela. Erdogan has economic and political problems, some of which he blames on Washington.

NATO member Turkey's resentment over what it considered insufficient U.S. and European concern about the threat it faced from the civil war in neighboring Syria led it to purchase Russia's S-400 air defense system, a violation of both U.S. law and alliance protocols. In response, the administration expelled Turkey from the F-35 program - a consortium of nations in which its defense and steel industry profited by providing aircraft parts. The United States also canceled Turkey's purchase of more than 100 of the planes.

Trump hinted broadly Tuesday that the problems could be rectified. "So many people conveniently forget that Turkey is a big trading partner of the United States," he tweeted, "in fact they make the structural steel frame for our F-35 Fighter jet. . . . Also remember, and importantly, that Turkey is an important member in good standing of NATO. [Erdogan] is coming to the U.S. as my guest on November 13th."

Covering a broad swath of ground and many audiences, Trump made his first mention of the Kurds in another tweet moments later. "In no way have we Abandoned the Kurds, who are special people and wonderful fighters," he wrote. "Likewise our relationship with Turkey, a NATO and Trading partner, has been very good."

Within seconds, however, he reverted to warning about devastation to the Turkish economy.

The piecemeal announcements appear to have added to the widespread criticism that administration policy on Turkey, as on so many other foreign fronts, is in disarray.

"The president is doing a good job, in his own way," said a senior U.S. official familiar with the twists and turns over Turkey this week. He had done, in the call, everything that was expected of him, the official said - discussing the incentives and expressing concern about any invasion.

"Everything you're seeing that we're doing in kind of a Wild West way with his tweets is more or less what he put on the table in a rational manner on the Sunday" call, the official said.

"We're trying to do two things," the official said. "We want to move forward" with a major package of incentives to repair the U.S.-Turkey relationship. "But we cannot have a disaster in northeast Syria."

Meanwhile, Turkey's Defense Ministry tweeted Tuesday that all its preparations for the operation were complete.

U.S. military officials said American forces in Syria, including about 50 Special Operations troops who have now been repositioned outside the safe zone, were waiting to see the extent of Turkey's offensive to assess how the overall U.S. mission there would be affected.

Outside experts have cautioned that a large-scale Turkish operation, if it precipitated a security breakdown at prisons holding Islamic State militants, could prompt a larger U.S. withdrawal from Syria. The American presence, which includes about 1,000 troops in northeastern Syria, is a lean force dispersed across a number of bases.

An administration official said Mazloum Kobane Abdi, the commander of the Kurdish-dominated Syrian Democratic Force, sent a message to U.S. forces in Syria saying he knew that the recent decisions were not theirs and that they would always be brothers.

"To say the military is very angry about this is an understatement," the official said.

Officials said they still were uncertain whether Turkish forces would conduct a symbolic feint inside the border - which they said could enable the U.S. troops to return to reactivate the safe zone - or would force their way deeper into Syria.

Early Wednesday morning, the Islamic State sought to take advantage of the focus on the border to the north by staging an attack shortly before 2 a.m. in the group's former capital, Raqqa, according to a statement from the SDF. Three suicide bombers attacked SDF military positions in the city, the statement said, and a gun battle erupted with an unknown number of militants nearby.

As allies and regional actors try to unscramble the president's conflicting statements, Erdogan's government has remained on message, insisting that the invasion is a certainty and that its target, the SDF, is an imminent threat to Turkey's national security.

Sabah, a Turkish newspaper that is close to Erdogan's government, published a report Tuesday describing how the battle might unfold. It said Turkish armed forces would wait for the full withdrawal of U.S. troops before commencing any operation. Warplanes and artillery would pound enemy positions, and then Turkish troops would enter Syria from several points along the border, east of the Euphrates River.

The military would advance as far as 18 miles into Syrian territory, the report said, without naming its source. After the operation was completed, Turkey would "continue its humanitarian work to bring back locals in the area."

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Fahim reported from Istanbul. The Washington Post's Sarah Dadouch and Liz Sly in Beirut and Missy Ryan in Washington contributed to this report.

This article was written by Karen DeYoung and Kareem Fahim, reporters for The Washington Post.