"Everyone was absolutely flabbergasted by this. I tell you that as a fact," retired Adm. James Stavridis said Monday on MSNBC, describing what he saw as the view from the Pentagon. "Nobody saw it coming, and that is a real problem when you're trying to conduct not only foreign policy ... but also military operations. That kind of whipsawing effect is extremely detrimental, not only in this tactical situation, but strategically as our planners try and prepare in other theaters, from North Korea to Afghanistan."

A person familiar with the U.S. government’s policy deliberations on the issue said Trump is operating against the advice of his national security leaders — noting that Sunday night's announcement came just three days after Defense Secretary Mark Esper spoke by phone with Turkish Defense Minister Hulusi Akar.

“POTUS went rogue," the person said. "It’s not too surprising for those of us who’ve been following him, but it was a surprise and went against what Esper was talking to Akar about.”

Last week, the Pentagon and Special Representative for Syria Engagement James Jeffrey, the State Department’s point man for the country, believed they had a system in place — a safe zone and program of joint patrols known as the “security mechanism” ⁠— to prevent the Turks’ long-threatened incursion against the Syrian Democratic Forces. Turkey views the U.S.-backed paramilitary group as a terrorist organization, since many of its members are part of Kurdish guerrilla groups that have fought the Turkish government.

On the phone call, Esper told reporters Friday, he and Akar agreed that the safe zone system was the best approach to securing the Turkish-Syrian border. “I made very clear to him, and he agreed as well, that we need to make the security mechanism work,” Esper said. “We have air patrols going on, we had another ground patrol just happen. … I just told him, let's keep working at it. That's the best path forward for all of us, so that's what I'm focused on right now.”

As of Saturday, Pentagon officials were still taking Esper's safe-zone comments as their guidance. “Indicators of Turkish military movements were pretty sparse — although folks were getting a little antsy about it," a Defense official said. "Obviously, things have changed over the past 24 hours.”

In a statement Monday, Pentagon spokesperson Jonathan Hoffman said that “the Department of Defense made clear to Turkey ⁠— as did the President ⁠— that we do not endorse a Turkish operation in Northern Syria” and will not be involved in it.

“Secretary Esper and Chairman [Mark] Milley reiterated to their respective Turkish counterparts that unilateral action creates risks for Turkey,” Hoffman added.

Jack Keane, a retired Army general who has advised Trump on national security issues, said he didn't believe Trump meant to endorse the Turkish move, and that on the phone call yesterday, as he understands it, Erdoğan told Trump that Turkey had no other option but military action — ignoring the safe zone system that has been getting underway.

“I don’t think the U.S. decision is endorsing Turkish military action by getting out of the way,” Keane said. “When Erdoğan brought up the Syrian issue on the phone call, I understand he presented it as if [Erdoğan] had no other choice, which is surprising given that negotiations were ongoing.”

Yet whether the U.S. endorses Turkish military action or is merely stepping out of the way will make little difference to the Syrian Kurdish troops who suffered thousands of casualties driving ISIS out of Syrian strongholds, said another defense official, who was not authorized to speak on the record about the situation. “I don’t see a difference. And more importantly, the Kurds don’t either,” the second official said.

The announcement Sunday night also surprised the State Department’s special representative for Syria, Jim Jeffrey, and his aides, according to the person familiar with the U.S. government's deliberations. Last week, Jeffrey and his aides “thought Turkey was bluffing, that the bluff was for domestic audiences, and that they could make the safe zone work,” the person said. “There was talk about how to manage Turkey, albeit without giving them a permanent presence,” including by informing Turkey that even after U.S. troops eventually left, U.S. aircraft would still patrol the region.

Those plans appear to be out the window after the Trump-Erdogan call, the person said.

But Keane suggested they might be salvaged as the Turkish military realizes the difficulties of fighting against the Syrian Democratic Forces ⁠— especially if those forces, lacking U.S. air support, instead turn to the Assad regime for protection.

“There was a pathway to a diplomatic solution, and I sense as the Turks look at this clear-eyed, there may be enough motivation to get us back on the pathway,” Keane said.

Quint Forgey contributed to this report.