Trump's Secretary of Homeland Security only learned about the Muslim ban executive order as Trump was actually signing it on television.

Homeland Security Secretary John F. Kelly

Courtesy the New York Times:

As President Trump signed a sweeping executive order on Friday, shutting the borders to refugees and others from seven largely Muslim countries, the secretary of homeland security was on a White House conference call getting his first full briefing on the global shift in policy. 

Gen. John F. Kelly, the secretary of homeland security, had dialed in from a Coast Guard plane as he headed back to Washington from Miami. Along with other top officials, he needed guidance from the White House, which had not asked his department for a legal review of the order. 

Halfway into the briefing, someone on the call looked up at a television in his office. “The president is signing the executive order that we’re discussing,” the official said, stunned.

As we know the executive order was drafted by Stephen Bannon and a small group of White House advisers, without any input from career officials at the Homeland Security Department, the State Department or other agencies.

The reason given by  James Jay Carafano, a vice president of the conservative Heritage Foundation, and also a member of Trump's transition team, is a fear of leaks.

 “Why share it with them?” Mr. Carafano said. 

Well part of the reason you "share it with them" is that they actually know the legalities of something like this, and could  have shut it down or at least given guidance that would have helped to avoid those incidents at the airports.

“The details of it were not thought through,” said Stephen Heifetz, who served in the Justice and Homeland Security Departments, as well as the C.I.A., under the previous three presidents. “It is not surprising there was mass confusion, and I expect the confusion and chaos will continue for some time.” 

Trump's distrust of the intelligence agencies and his reliance on loose cannons like Stephen Bannon will be his downfall.

The only question is will that happen before he irreparably damages this country?

P.S. By the way a number of Trump spokespeople are claiming that this ban is just like the one that President Obama imposed on Iraq for six months.

Problem is, that never happened.

Courtesy of WaPo: 

Former Obama administration official Jon Finer denied that any ban in Iraqi refugee admissions was put in place under Obama. “While the flow of Iraqi refugees slowed significantly during the Obama administration’s review, refugees continued to be admitted to the United States during that time, and there was not a single month in which no Iraqis arrived here,” he wrote in Foreign Policy. “In other words, while there were delays in processing, there was no outright ban.” 

Another former official, Eric P. Schwartz, the assistant secretary of state for population, refugees and migration at the time, also told The Fact Checker that Trump’s statement is false: 

“President Obama never imposed a six-month ban on Iraqi processing. For several months in 2011, there was a lower level of Iraqi resettlement, as the government implemented certain security enhancements. Indeed, as we identified new and valuable opportunities to enhance screening, we did so. Nobody should object to a continual effort to identify legitimate enhancements, but it is disreputable to use that as a pretext to effectively shut down a program that is overwhelmingly safe and has enabled the United States to exercise world leadership. In any event, there was never a point during that period in which Iraqi resettlement was stopped, or banned.”

The Post gave those statements by the Trump spokespeople three Pinocchios.