A nice illustration of the limits of congressional power. Congress passed the National Defense Authorization Act of 2014, which provided that if the president wants to transfer Guantanamo detainees to foreign countries, he must give Congress 30 days notice, so that it can raise a stink. The president refused to do so before the prisoner exchange since the negotiations took place in secret, and he could not afford the risk that even if he could inform Congress secretly, a leak would occur.
The transfer may have violated the substantive provisions of the statute as well. The statute provides that a detainee can be transferred only if he is no longer a threat to U.S. national security or the transfer is pursuant to a court order. But there was no such court order, and the five Taliban members who were transferred appear to be dangerous people. So much for the statute.
As Marty Lederman notes, the president paved the way with a signing statement that said that the law conflicts with “separation of powers principles.” A nice echo of the Bush administration, and a reminder that the imperial presidency is bipartisan.