As far as the US is concerned, it's fair game to drone-strike US citizens [0,1,2] from time to time.

[0] https://en.wikipedia.org/wiki/Anwar_al-Awlaki#Death

[1] https://en.wikipedia.org/wiki/Killing_of_Abdulrahman_al-Awla...

[2] https://en.wikipedia.org/wiki/Killing_of_Nawar_al-Awlaki