In 1948, a U.S. Army report to the Secretary of Defense warned that mixing races within military units would erode their effectiveness. “The soldier on the battlefield deserves to have the utmost confidence in his fellow soldiers,” the report warned. “They must eat together, sleep together, and all too frequently die together. There can be no friction in their everyday living that might bring on failure in battle.”

Sound familiar? It’s the same argument that President Donald Trump invoked last Wednesday, when he barred transgender people from the military. According to White House press secretary Sarah Huckabee Sanders, Trump concluded that allowing transgender people to serve in the armed forces “erodes military readiness and unit cohesion.”

But that’s flat-out false. Several thousand transgender people are already on active duty in the U.S. military, and there is no evidence that they harm “readiness” or “cohesion.” Those terms are stalking-horses for prejudice and discrimination, every bit as much as they were in 1948.

At least 16 other countries allow transgender people to serve in their armed forces. Canada’s military released a clever tweet shortly after Trump’s announcement, noting that it welcomes “Cnds of all sexual orientations and gender identities” and adding a pointed hashtag: “DiversityIsOurStrength.” In another jab at Trump, United Kingdom Maritime Forces commander Adm. Alex Burton tweeted that he was “so glad we are not going this way.”

So as Trump and his allies on the Christian right rally around the new transgender ban, we should ask them a simple question: How could it be wrong to segregate African-Americans in the military but OK to bar transgender people from it? ...