But turning the Bush and Obama administrations' interpretations of their war powers into an actual law is "no small thing," as Benjamin Wittes, a legal expert at the Brookings Institution,explains. Under this law, the government has far-reaching powers to detain and try terrorist suspects inside or outside the civilian justice system—or, if necessary, to transfer them to the custody of foreign powers—and it will serve as a signal to judges. "When you put all that in a statute, it becomes a much more permanent fixture of the US justice system," says Daphne Eviatar, a lawyer with Human Rights First. "It's not necessarily changing the authority the US government has today, but it's institutionalizing it."

Eviatar adds that there are "a whole lot of scenarios" where the government might want to transfer a suspected terrorist—even a US citizen—to foreign custody. For example, the administration might not want to go through the political mess of determining whether to send a suspect to Gitmo, try him in a military commission, or use the civilian system. The administration might also want to avoid the mandatory habeas corpus review that would come if the US held the suspect itself. In such a case, transferring the suspect to a foreign security force might present an appealing option.

An amendment sponsored by Sen. Dianne Feinstein (D-Calif.) says the relevant section of the NDAA doesn't change "existing law" with regards to the detention of US citizens, permanent legal residents, and terrorist suspects captured in the United States. The problem is that there's a debate over what "existing law" is.

Civil liberties advocates and some members of Congress argue that the government can only indefinitely detain an American if he is, as Feinstein explained on the Senate floor, "taken an active part in hostilities against the United States and is captured outside the United States in an area of 'active combat operations,' such as the battlefields of Afghanistan." 

But many legal experts, members of Congress, and both the Bush and Obama administrations have argued that existing law allows the US to indefinitely detain all people, including American citizens, who the president determines are part of or "substantially supported" Al Qaeda, the Taliban, or associated forces. (You can see an Obama administration argument on this subjecthere.) The Supreme Court has yet to issue a definitive ruling on the issue.