ANALYSIS: No one saw Trump's transgender military ban coming

— -- It's hard to overstate the enormity of President Donald Trump's decision today to ban transgender people from military service.

Not only does it deal a lasting blow to President Barack Obama's legacy of reversing discrimination within the Armed Forces, but it raises so many questions about how and why Trump chose to do this.

For instance: By all accounts, this announcement took everyone in Washington, D.C., by surprise. Neither the Pentagon nor the White House communications staff nor Congress were in any way prepared to answer questions about this. They clearly did not see it coming.

Last anyone heard about the issue was on June 30, when Defense Secretary James Mattis announced he was still reviewing the Obama-era policy that allowed transgender people to join the military (about a year ago Obama's defense secretary said no longer would the military treat "gender dysphoria" as a medical condition that precluded individuals from service). Mattis said then that he was giving his people until December to produce a report on the matter. There is no indication Trump had that review before he tweeted out his announcement this morning.

We also know there was a debate earlier this month in Congress about an amendment that would make it illegal for the military to pay for gender reassignment surgery. That amendment, thanks to many Republicans, failed. But that amendment wasn't a call to ban all transgender persons, it just maintained that the military shouldn't pay for related surgeries.

However, unless and until the White House or the Pentagon clarify -- which so far they haven't -- there is no other way to interpret Trump's statement today other than an absolute ban on transgender service.

Here's what he tweeted in full:

So does that mean all transgender people serving today will be discharged? Incoming White House press secretary Sarah Sanders had no idea when ABC News asked today. She punted to the Defense Department. The Defense Department then kicked the question back to the White House.

Translation: Nobody saw this coming and they don't know what to say.

Sanders said Trump made the decision after consulting with his national security team and said that it was based on issues of "military readiness" and concerns about "unit cohesion."

Readiness likely means they're worried about costs and unit cohesion means they think transgender persons could be a distraction in the battlefield. These were the same buzzwords used when some lobbied against the repeal of "Don't ask, don't tell."

Trump mentioned costs in his series of tweets as well, saying the military would be "burdened."

Yet the Rand Corporation, a reputable think tank that has for decades been providing research and analysis to the Defense Department, estimated in June that the total annual costs for the active duty would be between $2.4 to 8.4 million, or between .04 and .13 percent of active duty health care costs.

So that's probably not where he's getting his numbers. Family Research Council, a conservative Christian lobbying group, came up with different numbers. It called the Rand study faulty and says the medical costs plus the lost time for personnel in the field could cost taxpayers between $1.9 and $3.7 billion over 10 years.

Costs aside, polling shows by and large that Americans favor equal protection for transgender people.

According to a Public Religion Institute poll released in March 2017, 53 percent of Americans oppose laws that require transgender individuals to use bathrooms that correspond their sex at birth. A CNN/ORC poll from 2016 shows 75 percent of Americans favor laws that guarantee equal protection in housing and public accommodations for transgender people.

In any case, a policy like this can't be enforced via tweet. The commander-in-chief will have to take some other action, perhaps via executive order, to put any changes into effect. And for now, it's not clear when or how that will occur.