The Trump administration is making its strongest push yet for a ceasefire in northern Syria. Vice President Mike Pence and Secretary of State Mike Pompeo sat down with Turkish President Erdogan Thursday morning in Turkey's capital, Ankara. They're trying to convince Turkey to halt its military operation against the Kurds.

But Retired Admiral Sandy Winnefeld, a CBS News military and homeland security analyst and former vice-chairman of the Joint Chiefs of Staff, said even if they manage to attain a ceasefire, which Turkey has so far refused, there are several other problems at the strategic level with the situation.

"It's going to be difficult to fix this. It's essentially irreversible," Winnefeld told "CBS This Morning." "Because the territory that we're leaving is very rapidly going to be filled in by Syrian forces that are supported by Iranians and Russians. And the Kurds have switched their alliance over— over to that. So it's essentially irreversible."

Winnefeld said even if the U.S. sent troops back to the region, the ground they had gained would be gone.

"I don't think that we'll send troops back in unless perhaps there were a major terrorist attack against the United States or a close ally," Winnefeld said. "That ground is gone now. We handed it over for free."

In the wake of his authorization to pull U.S. troops from Syria, President Trump has said Americans don't belong in the region's conflicts and that the Kurds are actually safer now. Winnefeld had other thoughts.

"Several hundred Kurds, I think, have been killed in the last few days in the initial phases of this incursion. I don't believe that I can credibly say that the Kurds are safer now," Winnefeld said."

Mr. Trump also said repeatedly Wednesday the Kurds were "not angels." In response to that, Winnefeld cited the Kurds' history with the United States.

"We began our closest relationship with them in 1991 when we sent them humanitarian aid after the first Gulf War, and they never forgot that," Winnefeld said. "They've been very loyal allies and friends since, have supported us in numerous operations in Iraq. And now they've been fighting and dying to the tune of 11,000 Kurds who have essentially been working on the ground against ISIS, who is a sworn enemy of the United States."

Winnefeld also commented on the letter President Trump wrote to Erdogan, obtained by CBS News, asking him to handle the situation the "right and humane way." Mr. Trump wrote, "Don't be a tough guy. Don't be a fool."

"When I first saw the letter, candidly, I almost thought it might have been a fake," Winnefeld said. "And it didn't seem to have the gravitas that you would expect a superpower leader to use when speaking to an ally who's stepping out of line. But the president's entitled to his own style. It just seems a little bit unusual."