President Trump announced on Twitter Wednesday that he will ban transgender people from serving in the military in any capacity, reversing an Obama administration decision to allow them to serve openly and drawing dismay and anger from advocates.

Citing the need to focus on what he called “decisive and overwhelming victory,” Trump said that the military cannot accept the burden of higher medical costs and the “disruption” that transgender troops “would entail.”

“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,” Trump wrote on Twitter. “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.”

The missive comes after Trump’s defense secretary, retired. Gen. Jim Mattis, delayed the implementation of the last significant piece of the Obama administration’s new policy, delaying the entry of transgender military recruits until the end of 2017. The policy in place would have allowed them to begin serving July 1, but Mattis delayed it just before a deadline set, citing a need for more study.