Washington (CNN) President Donald Trump claimed Wednesday he "essentially" fired his former Pentagon chief James Mattis, despite a resignation letter in which Mattis wrote he was departing because he didn't share Trump's worldview.

"I wish him well. I hope he does well. But, as you know, President (Barack) Obama fired him and essentially so did I," Trump said. "I want results."

Mattis departed as head of Central Command under Obama, in part because of disagreements over Iran.

In his resignation letter to Trump, Mattis said the President deserved a defense chief more closely aligned with his "America First" views.

Trump did decide to move Mattis' departure date up to January 1, two months earlier than originally planned.

Aides said at the time that Trump was frustrated at coverage of Mattis' resignation.

Trump said during his Cabinet meeting on Wednesday that Mattis was thrilled when Trump secured hundreds of millions of dollars in military funding. But he said Mattis hadn't provided any success in return.

"What's he done for me? How has he done in Afghanistan? Not too good. Not too good. I'm not happy with what he's done in Afghanistan and I shouldn't be happy," Trump said.

Mattis' former deputy, acting Secretary of Defense Patrick Shanahan, sat at the table alongside Trump throughout the meeting and listened quietly to the President's remarks.

A source close to Mattis told CNN that in the past he has referred to Shanahan as "my co-secretary of defense" and praised him as someone whose input was sought for every "decision, document and strategic issue."

The source also said Mattis personally likes him, and has said "still waters run deep" with Shanahan.

After recalling a meeting in which he questioned why the Taliban could not fight ISIS without US involvement, Trump mused he might have made a good military man.

"I think I would have been a good general, but who knows?" Trump said. While of the Vietnam era, Trump himself did not serve after receiving a draft exemption for bone spurs.

He went on to defend his decision to withdraw US troops from Syria -- which was a facet of Mattis' decision to resign -- saying the country was not worth the US effort.

"We're talking about sand and death. We're not talking about vast wealth. We're talking about sand and death," he said.

Trump said his withdrawal of troops from Syria would be neither fast nor slow but declined to specify a timeline for bringing home the military.

"Somebody said four months. I didn't say that," he said, adding later: "I think it's just over a period of time."

"We're getting out of Syria. Look, we don't want Syria," Trump said, before knocking his predecessor, Obama, for deciding "not to violate his statement" about the red line of Syria using chemical weapons on civilians.

"You know making a threat is OK, but you always have to follow through with it," Trump said, adding, "Syria was lost long ago."

The President revived his misleading claim that Russia is unhappy with the American troop withdrawal from Syria because it forces Russian troops to take over the fight against ISIS.

"Russia is not happy. You know what? They're not happy," Trump said. "We're killing them for them. ... We're killing ISIS, also, for Iran."

"I never said we're getting out tomorrow," the President added. "I said we're pulling our soldiers out. ... But we're getting out very powerfully."

He also talked about Kurdish fighters in Syria, who have been backed by the US. Republican Sen. Lindsey Graham of South Carolina has warned the President those fighters could be slaughtered if American forces leave the country.

"The Kurds, it's very interesting," Trump said. "Turkey doesn't like them; other people do."

"I didn't like the fact that they sell the small oil that they have to Iran," the President said. "We're not thrilled about that."

"At the same time, they fight better when we fight with them."

"We want to protect the Kurds nevertheless," Trump said. "But I don't want to be in Syria forever."