In a letter to Congress, Lt. General William Mayville Jr., the director of the Joint Staff, made a move to defy President Barack Obama by stating that if the Guantanamo Bay Detention Facility were to be shut down by the president, the U.S. military would not cooperate in the transfer of the detainees to the mainland United States, according to Bloomberg View.

Mayville's letter, sent to Congress last week and obtained by Bloomberg View's Eli Lake, addresses the issue of the constitutionality of the president's authority to close Guantanamo without Congress agreeing. Last December, Obama stated that "We will wait until Congress has definitively said no to a well-thought-out plan with numbers attached to it before we say anything definitive about my executive authority here," in regards to his intention to close the facility, reported the Conservative Tribune.

Mayville writes that "Current law prohibits the use of funds to 'transfer, release or assist in the transfer or release' of detainees of Guantanamo Bay to or within the United States, and prohibits the construction, modification or acquisition of any facility within the United States to house any Guantanamo detainee. The Joint Staff will not take any action contrary to those restrictions," according to Lake.

Defense Secretary Ash Carter sent a plan to the White House on how to close Guantanamo Bay last month, with the plan including the transfer of detainees to U.S. prisons. "Not everyone in GTMO can be safely transferred to another country, so we need an alternative," Carter said in January, according to The Washington Free Beacon.

Since 2009, Obama has transferred 151 detainees to other countries, but Congress prohibited detainees to be transferred to U.S. territories since 2010, explained Bloomberg View. There are currently 91 detainees still at the facility, including Khalid Sheikh Mohammed, the 9-11 mastermind, and Abu Zubaydeh, a top al-Qaeda planner.

Obama has promised to close the detention facility by the end of the year and plans to present a plan to do so to Congress in time. Whether or not he will need to use executive action remains to be seen, and if it comes to an executive order, the legality of that remains "unclear," according to Raha Wala, senior counsel for defense and intelligence at Human Rights First, reported Bloomberg View.

The Joint Staff supports closing down Guantanamo, Mayville notes, but only if the facility is closed by way of Congressional legislation, according to the Conservative Tribune.