Members of Congress, led by House Speaker Nancy Pelosi, were set to depart for a co-delegation (codel) trip to Brussels and Afghanistan Thursday afternoon. But shortly before the codel was set to depart, President Trump abruptly announced that he was postponing the trip by denying Pelosi use of U.S. military transportation. Mr. Trump's decision came the day after Pelosi asked Mr. Trump to delay the State of the Union due to security concerns.

"Due to the shutdown, I am sorry to inform you that your trip to Brussels, Egypt, and Afghanistan has been postponed," Mr. Trump wrote in a letter. "We will reschedule this seven-day excursion when the shutdown is over." The letter was released shortly before 3 p.m., when Pelosi and the codel were set to depart.

It is unusual for such detail about a codel to be revealed before departure, for security reasons. Virginia's senators, both Democrats, said they were stunned by the revelation of her trip stops before she left. "I have been to Afghanistan on multiple occasions. It has never been announced that I'm going to Afghanistan, that never happens, never," Sen. Tim Kaine said emphatically. Senior Virginia Sen. Mark Warner agreed. "If someone had revealed about the president's trip beforehand around Christmas that he went to a war zone...that would have been a violation of law, policy, and procedure," Warner said.

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Usually, these trips are announced after liftoff or when officials have landed, because of security concerns. The speaker's trip was a closely held secret, and a source familiar with the delegation's plans argued it was irresponsible and reckless for Mr. Trump to tell the world Pelosi was headed to a war zone — particularly since the president was in Iraq in December and saw how much care had to be taken to keep the trip secret and secure. Mr. Trump told reporters at the time that previous trips had been canceled for "security reasons" because "people were finding out" about them.

"Pretty sad when you spend $7 trillion in the Middle East and going in has to be under this massive cover, with planes all over and all of the greatest equipment in the world, and you do everything to get in safely," Mr. Trump said, referring to the high level of security for his own trip.

A bus full of House Democrats departed the Capitol this afternoon, and then abruptly turned around. Rep. Stephen Lynch confirmed that the group had been on its way to depart for the trip.

A senior White House official told reporters Thursday afternoon that the Pentagon had been informed before Mr. Trump sent the letter, and Pelosi had received it before it was released to the press. The official also said that the postponement was "not a response" to Pelosi's request to delay Mr. Trump's State of the Union address at the end of the month.

Drew Hammill, a spokesperson for Pelosi said in a statement that the codel had not planned a stop in Egypt. He also noted that Mr. Trump had visited U.S. troops in Iraq during the shutdown.

"The purpose of the trip was to express appreciation & thanks to our men & women in uniform for their service & dedication, & to obtain critical national security & intelligence briefings from those on the front lines," Hammill wrote on Twitter. Mr. Trump had referred to the trip in his letter as a "public relations" stunt.

For now, a delegation of Trump administration officials is still expected to attend the World Economic Forum in Davos, Switzerland, later this month. The president had also planned to attend the forum but canceled his plans because of the shutdown.