In this occasional series, we will bring you up to speed on the biggest national security stories of the week.

On Wednesday, President Trump announced on Twitter an abrupt and surprising policy change: He would reverse the Obama administration’s decision to lift the ban on transgender people serving in the U.S. military. They would not be allowed in the military “in any capacity” because of the supposed costs associated with providing them medical services and the “disruption” he said they would cause.

Trump’s decision seemed to surprise both the Pentagon and Congress, and several Republican lawmakers publicly disagreed with the move, saying anyone who meets military standards should be allowed to serve. The following day, the nation’s highest-ranking military officer sent a memo to military leaders saying that the policy remains unchanged until Defense Secretary Jim Mattis reviews Trump’s “direction” and guidelines are set on how to implement it.

Here’s what you need to know:

Is it true that transgender troops pose a “disruption” to the military?

The apparent reversal of Pentagon policy, issued while Mattis was away from Washington on personal travel, brings back a long-running debate within the Defense Department. The question is whether anyone who meets requirements can serve, or if there are too many difficulties involved in integrating transgender people in the military.

Trump cited two of the most common arguments used by some service members and veterans — cost and disruption to unit cohesion and readiness — but the “disruption” case in particular is weakened by the relatively seamless way that gay and lesbian troops were integrated after the end of the “don’t ask, don’t tell” policy in 2011.

Defense Department officials have said that hundreds of transgender service members have come out to their units in the year since President Barack Obama’s administration repealed the ban on them serving — and no significant problems have been reported. Several thousand more are believed to be serving, according to a study commissioned by the Pentagon.

A policy reversal actually creates a new kind of disruption: Depending on the specifics of what Trump wants, the Pentagon might face an exodus of hundreds of more service members, some of whom identified themselves last year after being told it was safe to do so. Along with that might come lawsuits that drag out for months, if not years.

How much does it cost the military?

Incoming White House press secretary Sarah Huckabee Sanders defended Trump’s decision on Wednesday, saying that allowing transgender people to serve in the military is “expensive and disruptive.”

But one of the authors of a RAND Institute report on transgender service points out that medical costs for transgender troops are about “one-tenth of 1 percent of medical costs” at the Defense Department.

“You have only a few thousand out of 1.3 million [active members],” said Radha Iyengar. “Of those, only a fraction are seeking extreme or invasive treatments. I’m talking between 10 to 130 service members seeking medical treatments that might affect their ability to deploy.”

Does allowing transgender troops to serve disrupt unit cohesion?

Experts say no. Iyengar, the author of the RAND study, said: “We’ve had commanders report that having a more improved attitude toward inclusiveness and diversity was beneficial to their unit overall. And that really speaks to the benefits, but there was really no effect, in any of the cases, of operational effectiveness. They saw these positives, but we really didn’t see any of the negatives.”

Note: A version of this brief appeared in Today’s WorldView, a daily newsletter that explores where the world meets Washington. Sign up here.