(CNN) The US military has been ordered to begin planning to withdraw about half the troops in Afghanistan, a US defense official with direct knowledge of the matter told CNN on Thursday.

The official said planning is underway, and it could take months to withdraw the nearly 7,000 troops.

The decision was made Tuesday, at the same time as President Donald Trump's decision to withdraw the US military from Syria -- moves that precipitated Defense Secretary James Mattis' resignation announcement on Thursday.

The Wall Street Journal was first to report the plan for the Afghanistan withdrawal.

Several US defense officials told CNN that Trump wants to draw down US troops in Afghanistan. Two administration officials told CNN that Trump wants the plans drawn up in hopes he could announce the drawdown in his State of the Union speech, which is traditionally at the end of January or early February.

CNN reported earlier Thursday that officials throughout the administration were bracing themselves for Trump to make an announcement about the US presence in Afghanistan.

Multiple officials told CNN the military decisions were a factor in Mattis' decision to resign.

Gen. John Allen, a former commander of NATO and US forces in Afghanistan, told CNN on Thursday that a drawdown in Afghanistan would be a mistake.

"Pulling out right now, just the announcement would create chaos in the strategy," Allen said.

The US has about 14,000 troops in Afghanistan, most of which are present as part of a larger NATO-led mission to train, advise and assist Afghan forces. Any withdrawal would be complicated by the fact that the United States is part of NATO's Resolute Support mission.

Trump has long been critical of the US presence in Afghanistan, which began after the September 11, 2001, terrorist attacks. But lawmakers have echoed Allen's concern about a hasty departure.

South Carolina GOP Sen. Lindsey Graham told CNN's Kate Bolduan on Thursday that "according to our military commanders and everybody I know, we want to withdraw from Afghanistan with honor and do it based on conditions on the ground."

"Based on my assessment in Afghanistan, if we withdrew anytime soon, you would be paving the way for a second 9/11," Graham said.

Graham noted that Trump has said he wants other countries to do the fighting.

"Since August of 2017, 5,600 Afghans have died fighting the Taliban and ISIS," he told Bolduan. "Eighteen American killed in combat, four killed through accidents. God bless the 22."

Just two weeks before the news of the withdrawal plan, the chairman of the Joint Chiefs of Staff, Joseph Dunford, said at a Washington Post Live event that he "had not recommended" that the US withdraw.

"I had not recommended that we leave Afghanistan because, again, in my judgment, leaving Afghanistan not only would create instability in South Asia, but in my judgment would give terrorist groups the space within which to plan and conduct operations against the American people, our homeland and our allies," Dunford said. "And that really is the problem we are trying to solve."

The same day news broke of the Afghanistan decision, the Department of Defense released a report on "enhancing security and stability in Afghanistan."

The report cited a ceasefire, statements of religious scholars and recent talks with the Taliban as evidence of progress.

"The combination of military escalation and diplomatic initiative have made a favorable political settlement more likely than at any time in recent memory," the report said.

Trump has long questioned troop presence in Afghanistan

Trump has repeatedly questioned the need to spend US blood and treasure in Afghanistan, asking in 2011: "When will we stop wasting our money on rebuilding Afghanistan? We must rebuild our country first."

When will we stop wasting our money on rebuilding Afghanistan? We must rebuild our country first. — Donald J. Trump (@realDonaldTrump) October 7, 2011

Since his election, the President has made his frustration with the continued military presence clear.

Outlining his strategy for the country in an August 2017 address , the President said, "I share the American people's frustration. I also share their frustration over a foreign policy that has spent too much time, energy, money -- and, most importantly, lives -- trying to rebuild countries in our own image instead of pursuing our security interests above all other considerations."

In an interview with The Washington Post last month, Trump laid out his rationale for keeping US troops in the country in a way that made clear the impetus to remain wasn't his.

"We're there because virtually every expert that I have and speak to say if we don't go there, they're going to be fighting over here," he said.

Nearly two decades in, a "stalemate"

More than a year after Trump announced his administration's new strategy for achieving success in Afghanistan and the wider region, the situation remains decidedly mixed, with the conflict at a "stalemate."

Though the casualty rate for US troops is far lower than it was earlier in the war, Americans are still losing their lives 17 years after it began. While the Taliban is unable to take major cities or towns, the Afghan security forces, despite receiving US support, are still unable to put an end to the insurgency.

US troops have continued to suffer casualties this year, even though they are largely serving in a supporting role, with local Afghan forces doing most of the fighting.

Afghan President Ashraf Ghani recently announced that about 29,000 Afghan soldiers and police had been killed or wounded since 2015. US casualties during that same period declined sharply as American soldiers largely shifted away from direct combat.

"We used the term stalemate a year ago, and, relatively speaking, it has not changed much," Gen. Joseph Dunford, the Chairman of the Joint Chiefs of Staff, told a conference in Halifax last month.

News about the Afghanistan plan comes a day after Trump ordered the "full" and "rapid" withdrawal of US troops from Syria, declaring the US has defeated ISIS in Syria.

The decision, a sharp reversal from previously stated US policy, surprised foreign allies and US lawmakers, sparking angry rebukes, rebuttals and warnings of intensified congressional oversight even as the White House said troops are already on their way home.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," Trump tweeted Wednesday morning.

Planning for that pullout is already underway, a US defense and an administration official told CNN.