summarily execute them as spies
send them to an American tribunal for adjudication
treat them as prisoners of war despite not being in uniform
hand them over to the authorities of whatever nation they are captured in
send them for "rendition" in an allied nation
8 U.S. Code § 1481
I don't think it would apply here because ISIS is not a recognized state.(a) A person who is a national of the United States whether by birth or naturalization, shall lose his nationality by voluntarily performing any of the following acts with the intention of relinquishing United States nationality—
(1) obtaining naturalization in a foreign state upon his own application or upon an application filed by a duly authorized agent, after having attained the age of eighteen years; or
(2) taking an oath or making an affirmation or other formal declaration of allegiance to a foreign state or a political subdivision thereof, after having attained the age of eighteen years; or
(3) entering, or serving in, the armed forces of a foreign state if
(A) such armed forces are engaged in hostilities against the United States, or
(B) such persons serve as a commissioned or non-commissioned officer; or
(A) accepting, serving in, or performing the duties of any office, post, or employment under the government of a foreign state or a political subdivision thereof, after attaining the age of eighteen years if he has or acquires the nationality of such foreign state; or
(B) accepting, serving in, or performing the duties of any office, post, or employment under the government of a foreign state or a political subdivision thereof, after attaining the age of eighteen years for which office, post, or employment an oath, affirmation, or declaration of allegiance is required; or
(5) making a formal renunciation of nationality before a diplomatic or consular officer of the United States in a foreign state, in such form as may be prescribed by the Secretary of State; or
(6) making in the United States a formal written renunciation of nationality in such form as may be prescribed by, and before such officer as may be designated by, the Attorney General, whenever the United States shall be in a state of war and the Attorney General shall approve such renunciation as not contrary to the interests of national defense; or
(7) committing any act of treason against, or attempting by force to overthrow, or bearing arms against, the United States, violating or conspiring to violate any of the provisions of section 2383 of title 18, or willfully performing any act in violation of section 2385 of title 18, or violating section 2384 of title 18 by engaging in a conspiracy to overthrow, put down, or to destroy by force the Government of the United States, or to levy war against them, if and when he is convicted thereof by a court martial or by a court of competent jurisdiction.
(b) Whenever the loss of United States nationality is put in issue in any action or proceeding commenced on or after September 26, 1961 under, or by virtue of, the provisions of this chapter or any other Act, the burden shall be upon the person or party claiming that such loss occurred, to establish such claim by a preponderance of the evidence. Any person who commits or performs, or who has committed or performed, any act of expatriation under the provisions of this chapter or any other Act shall be presumed to have done so voluntarily, but such presumption may be rebutted upon a showing, by a preponderance of the evidence, that the act or acts committed or performed were not done voluntarily.
"He who does not think himself worth saving from poverty and ignorance by his own efforts, will hardly be thought worth the efforts of anybody else." -- Frederick Douglass, Self-Made Men (1872)
If it cannot be proven they'd taken up arms against the US or allies, strip them of citizenship and give them a plane ticket to wherever they want to go. If they did take up arms, hold them as POW's until the war on terror ends.
"To waste, to destroy, our natural resources, to skin and exhaust the land instead of using it so as to increase its usefulness, will result in undermining in the days of our children the very prosperity which we ought by rights to hand down to them."~ Theodore Roosevelt (Message to Congress, Dec. 3, 1907)
Try them for treason, then execute them.
"I have never understood why it is "greed" to want to keep the money you have earned but not greed to want to take somebody else's money." -Thomas Sowell
If captured on the battlefield and are not wearing the uniform of a sovereign state, execution by musketry.
Any American who's waging war against America or an ally should be tried by a military court for treason and if found guilty, execution by musketry.