Is it allowable for an Islamic nation to behead prisoners it captures in a war (like how ISIS has captured and beheaded an American)? Is there any Quranic or hadith justification for this?
Well I’m not sure about the specific hadith or Qur’anic justification but IF the prisoner has committed the crime punishable by beheading then yes, the state can punish the prisoner according to its law! BUT the punishment must be from the ruling state or the proper authority (courts etc. depending upon the state law), and not from any individual who takes law in his/her own hands.
That was my personal understanding of your question and the answer is what i think is the best answer... again, I am not a Muslim scholar or Molvi. It's just my personal observation. Hope it helped.
بسم الله الرحمن الرحیم
In the name of Allah the merciful the compassionate
Islamic nation should act based on Islamic teachings. There is no verse or hadith explicitly allowing beheading of war prisoners. As for historical proof, we see that the prophet never beheaded any one when he conquested Mecca, although they were clearly idolaters (he only ordered to kill some of them who were murderers and criminals). In fact he was very kind to the slaves. He said:
Give them of the same food of which you eat yourselves, and provide them with the same kind of clothes as you put on yourselves. Do not force them to do jobs which are too difficult for them. Share with them their jobs and help them in the performance of them. Do not call them slaves, for all men are the bondsmen of Allah, who is the real Master of everyone. Call your male slaves young man and your female slaves young woman (1)