On Wednesday morning, Donald Trump announced in a series of tweets that he would bar transgender people from serving openly in the United States military. As justification, Trump cited “the tremendous medical costs and disruption that transgender in the military would entail.” The Pentagon was reportedly not briefed about Trump’s decision in advance.

After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow...... — Donald J. Trump (@realDonaldTrump) July 26, 2017

....Transgender individuals to serve in any capacity in the U.S. Military. Our military must be focused on decisive and overwhelming..... — Donald J. Trump (@realDonaldTrump) July 26, 2017

....victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail. Thank you — Donald J. Trump (@realDonaldTrump) July 26, 2017

In fact, the cost of transgender service is known to be quite low, and the integration of transgender troops in foreign militaries has had no effect on unit cohesion, morale, or force readiness. That is why, in 2016, then–Secretary of Defense Ash Carter established a plan to let openly transgender individuals enroll in the military and allowed already-serving transgender troops to come out without fear of discharge. The enrollment part of the plan was meant to be “completely implemented no later than July 1, 2017.” Once Trump took office, though, religious conservatives lobbied him to bar trans troops, arguing that they posed an immoral and expensive threat to the military. The anti-LGBTQ Family Research Council and some congressional Republicans reminded Trump that his socially conservative base opposed Carter’s policy. In late June, Secretary of Defense James Mattis approved a six-month delay, citing the “views of the military leadership and of the senior civilian officials now arriving in the Department.”

At that point, it was clear that Carter’s plan was in serious trouble. On July 13, the House of Representatives narrowly voted down an amendment that would bar the military from providing transition-related medical treatment to transgender troops. Vice President Mike Pence championed the amendment but Mattis, who supports trans service, vigorously lobbied against it. Two dozen Republicans ultimately opposed the measure, drawing ire from evangelicals and anti-LGBTQ activists. (Every Democrat also voted no.) It seems likely that following this vote, Trump realized he could seize upon the issue to rally his base.

In his tweets, Trump claimed that “my Generals and military experts” encouraged him to ban trans troops. I would like to know exactly which generals and which experts, because Mattis is not alone in his support for open transgender service: The policy enjoyed widespread support throughout the military, especially in light of the successful repeal of Don’t Ask, Don’t Tell. Opponents of allowing gay service members to serve openly cited the same concerns we are hearing now about unit cohesion, morale, and readiness, yet these fears turned out to be utterly unfounded.

Perhaps the most devastating aspect of Trump’s announcement is the uncertainty it foists upon active transgender service members. Thousands of transgender troops are already serving openly as the military lifted the retention ban a year ago. But Trump purports to have outlawed open transgender service entirely, tweeting that “the United States Government will not accept or allow Transgender individuals to serve in any capacity in the U.S. Military.”

In theory, this means that the military may soon discharge the roughly 15,000 transgender troops in uniform—although Trump may have only meant that he is maintaining the ban on transgender enrollment. (Who knows if anyone informed him of the difference between retention and enrollment?) We’ll find out soon enough. But for now, there is a real possibility that the commander in chief is attempting to purge thousands of men and women in uniform from the armed forces solely on account of their gender identity and in direct opposition to his own secretary of defense. This is an act of astonishing and capricious cruelty. And it is likely to be the worst day of thousands of Americans’ lives.