Washington (CNN) Over the past few days, the political gridlock that's led to the longest government shutdown in US history has devolved into a tit-for-tat between House Speaker Nancy Pelosi and President Donald Trump.

On Wednesday, Pelosi all but disinvited Trump from giving the State of the Union address in the House chamber, initially citing security concerns and suggesting the President wait until the shutdown was over. On Thursday, Trump hit back and canceled Pelosi's undisclosed plan to visit US troops in Afghanistan along with a congressional delegation, effectively blowing the cover for what is normally a secret trip.

Here's a look at the factors surrounding Trump's move to keep Pelosi and other lawmakers from using military transport during the shutdown.

Does this have anything to do with the shutdown?

Not directly. The Department of Defense has been fully funded since September and the trip would not necessarily require furloughed employees affected by the shutdown to come along.

In his letter, Trump argued that the trip would prevent Pelosi from entering negotiations to end the shutdown. (It should be noted that the President visited Iraq in late December during the shutdown.)

Is Trump allowed to do this?

Pretty much. When it comes to military airlift support for congressional delegations (CODEL) the assistant secretary of defense for legislative affairs -- with a few exceptions -- has to sign off . But, the president can, as we have seen, intervene and deny the request since, as commander in chief, he has authority over the military.

As announced in a memo sent out from the Office of Management and Budget on Friday morning, all congressional delegations are now prohibited from using any "government owned, rented, leased, or chartered aircraft" without the approval of acting White House chief of staff Mick Mulvaney.

Could she fly commercial?

Not really. In his letter to Pelosi, Trump said that she was free to fly commercial. And, according to her press office, she tried to.

Overnight, the State Department updated its security threat assessment for the trip, according to Pelosi's office , deeming it to be more dangerous because of Trump's decision to make the trip public. Citing these security concerns, the delegation canceled the trip.

Because the Speaker of the House is second in line for presidential succession, behind the vice president, military security is often required for travel to high-risk areas even if the speaker is able to fly commercial. Regardless, if one of the stops on the trip includes a visit to US military personnel stationed in a war zone like Afghanistan, the speaker can't go without military assistance.

Has this ever happened before?

Yes and no.

We could not find another instance where a president had singled out particular members of Congress and denied their CODEL request. It's also unprecedented that a president would alert the public to CODEL travel plans to a war zone, which according to Pelosi spokesman Drew Hammill, raised the State Department's threat level in the trip's security assessment.

There is precedent for speakers preventing particular members of Congress from traveling overseas. In 2015, Speaker John Boehner, an Ohio Republican, disapproved Texas GOP Rep. Louie Gohmert's planned trip to Egypt and Africa. (Boehner did the same to Republican Rep. Steve King of Iowa earlier that year.)

In the past, members of Congress have been limited in their ability to travel to certain countries due to military operations in the area and resource concerns.

According to retired Rear Admiral and CNN analyst John Kirby, lawmakers were temporarily denied CODEL requests to war zones during President Barack Obama's tenure due to sequestration and security concerns from military officials. Though, Kirby noted, the ban applied to both Republicans and Democrats and did not single out a particular member of Congress, as Trump did before his administration issued a blanket ban.