Fifty-six retired generals and admirals have a message for the president: His proposal to ban transgender troops would weaken the U.S. military, not make it stronger.

“This proposed ban, if implemented, would cause significant disruptions,” the retired generals wrote in a letter released Tuesday, “and compromise the integrity of transgender troops who would be forced to live a lie, as well as non-transgender peers who would be forced to choose between reporting their comrades or disobeying policy.”

Trump’s sudden, three-part tweet last Wednesday about the ban caught the Pentagon off-guard and plunged the future of thousands of active-duty transgender service members into uncertainty. The following day, the Department of Defense pushed back against the president’s abrupt proclamation reversing an Obama-era policy.

“We don’t have guidance. We have a tweet,” Pentagon spokesperson Capt. Jeff Davis said. “We don’t execute policy based on a tweet.” The Pentagon told VICE News Tuesday that it still hasn’t received any further information from the Trump administration about the ban.

Hours after Trump’s tweets, Deputy Press Secretary Sarah Huckabee explained that Trump believed transgender troops were disruptive and expensive. “This was about military readiness,” Huckabee told reporters. “This is about unit cohesion.”

“We respectfully disagree and consider these claims to be without merit,” the retired generals and admirals wrote Tuesday. Their letter cites Pentagon-commissioned research by the RAND Corporation and the New England Journal of Medicine that estimates the annual cost of providing healthcare to transgender service members to be about $8.4 million — one-hundredth of one percent of the military’s annual healthcare budget.

“As for ostensible disruptions, transgender troops have been serving honorably and openly for the past year,” the letter continued. The Obama administration last year ended the ban on transgender individuals serving in the military.

Separately, former Chairman of the Joint Chiefs of Staff Adm. Mike Mullen compared the ban on transgender troops openly serving to the “flawed ‘Don’t ask, don’t tell’ policy” (which the Obama administration also repealed in 2011). Mullen said he “saw firsthand the harm to readiness and morale when we fail to treat all service members according to the same standards.”

Some active-duty generals and officials have also taken a public stand. The commandant of the Coast Guard said Monday that he had no intention of leaving transgender Coast Guard personnel high and dry.

“I will not turn my back,” said Adm. Paul Zukunft during remarks at the Center for Strategic and International Studies. “We have made an investment in you, and you have made an investment in the Coast Guard, and I will not break faith.”

The letter, however, is not necessarily representative of the shared position held by former military officials. There are more than 4,000 generals and admirals enjoying retirement; 56 is just a tiny fraction. Still, a Reuters/Ipsos poll found that 58 percent of Americans supported transgender troops in the military, while 27 percent were actively opposed to the idea.

The Obama administration faced heavy criticism from some top-ranking military personnel when it first announced its policy change regarding transgender troops. Some accused President Obama of social experimentation at the expense of military readiness. Even some conservative lawmakers weighed in.

“How about having military focusing on hunting down and killing the bad guys,” Texas Republican Sen. Ted Cruz said, “instead of treating it as this crucible for social justice innovations. We’ve lost sight of what their job is.”