WASHINGTON, D.C. — The Pentagon was blindsided by President Donald Trump’s announcement on Wednesday that his administration would block transgender people from the U.S. military, and the Defense Department has no idea yet how it will affect troops already serving.

The president’s declaration on Twitter, saying transgender people would not be allowed to serve “in any capacity,” came a year after the Defense Department under former President Barack Obama lifted its ban on transgender troops serving openly.

On Wednesday, neither the Pentagon nor the White House could answer how the Trump administration intends to carry out such a ban — announced while Defense Secretary Jim Mattis was on vacation — or what it means for the thousands of transgender people already serving in the military.

The sudden blanket ban on all transgender troops seemed to take everyone by surprise in its scope. Lawmakers had been debating funding for medical care for transgender troops, and military leaders had been analyzing the impact of allowing transgender recruits. But no one had been debating a reversal of the existing policy allowing transgender troops to serve in the military.

Trump’s language indicated that those currently serving could be forced out, which advocates condemned as a betrayal after transgender troops were encouraged to identify themselves and serve openly after last year’s policy change. Military LGBT groups and civil rights groups threatened to sue if troops aren’t allowed to serve based on gender identity.

Indeed, reaction was swift, and criticism came from both Republicans and Democrats.

Trump’s tweet “is yet another example of why major policy announcements should not be made via Twitter,” said Sen. John McCain, R-Ariz., who chairs the Senate Armed Services Committee.

McCain, who came to Trump’s aid Tuesday by returning to Washington after his cancer diagnosis to cast a critical health care vote, said any military personnel policy change should only come after a study had been “thoroughly reviewed by the secretary of defense, our military leadership, and the Congress.”

“The statement was unclear,” McCain said of Trump’s tweet. “There is no reason to force service members who are able to fight, train, and deploy to leave the military — regardless of their gender identity.”

The president’s tweets sent the Pentagon, which was clearly not prepared to roll out any new guidance, scrambling. A few hours after being caught off guard by the announcement, a Pentagon spokesman would only say that the military would “work closely with the White House to address the new guidance provided by the commander in chief,” which could be expected “in the near future.”

The Pentagon referred all questions to the White House, which did not provide any further details.

Asked what would happen to transgender members of the military currently deployed to war zones, White House press secretary Sarah Huckabee Sanders could not answer. She insisted the move had been based on a “military decision,” but did not explain how it would be implemented or how it would impact active duty troops.

When the questions did not let up, she threatened to “call it a day” if reporters kept pressing for details.

Trump informed Mattis of his decision Tuesday, Sanders said. The Pentagon would not say whether the Pentagon chief agreed with it. Mattis had just recently delayed his decision by six months to “evaluate more carefully the impact” on military readiness of allowing transgender people to enlist. It was unclear why the president pushed forward Wednesday, especially as it wasn’t high on the Pentagon’s priority list.

As of Wednesday evening, the Defense Department website still carried the Obama-era policy, stating: “effective immediately, transgender service members may serve openly, and they can no longer be discharged or otherwise separated from the military solely for being transgender individuals.”

There are between 1,320 and 6,630 transgender individuals currently serving on active duty, about 0.05 percent of the total active force, according to a Rand Corp. analysis. Other reviews put that number as high as 15,000.

Last year the Defense Department commissioned an extensive Rand study that concluded that letting transgender people serve openly would have a “minimal impact” on both military readiness and health care costs.

“To say that the impact on readiness or medical costs are too high is just not consistent with the data,” Radha Iyengar, a senior economist at the Rand Corp. who authored the report for the Pentagon, told McClatchy on Wednesday.

The analysis found that the cost could range from $2.4 million to $8.4 million, which would make up an “exceedingly small proportion” of total health care expenditures. If, as the president indicated, the medical costs are a concern, he should consider the much larger costs associated with discharging and replacing military personnel, Iyengar said.

“There are also the costs to not allowing them open service, costs to separation that are real,” she said. “We spend a lot to train and equip them, and losing that is problematic.”

It cost the U.S. military $52,800 to discharge and replace each service member fired under “don’t ask, don’t tell,” according to a 2011 report from the Government Accountability Office. With more than 6,000 transgender service members on active duty, according to Rand Corp., what Trump is proposing would be both inconvenient and expensive.

The study estimated that there would only be 30 to 140 new hormone treatments a year in the military, with 25 to 130 gender transition-related surgeries among active service members. That number is “negligible and significantly smaller than the lack of availability due to medical conditions,” the study said. In 2015, there were 102,500 nondeployable soldiers in the Army alone.

In his tweets, Trump said the decision had been made “after consultation with my generals and military experts.”

“Please be advised that the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military,” the president tweeted. “Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail.”

Since October, transgender troops have been able to receive gender transition medical care through military services, and begin to formally change their gender identification in the Pentagon’s personnel system. The Obama administration had set a deadline of July 1, 2017, for the Pentagon to develop guidelines to allow transgender individuals to join the military if they met physical and medical standards.