What if someone in Greenland sniped an US Marine in the head and kills him, what happens?

The US Marine is there illegally and invading a country he is not from. I think it is Justifiable homicide. How can US forces take a person in to custody if he is defending his country?

What if someone in Greenland sniped an US Marine in the head and kills him, what happens?
US Marine shouldn't be there , justified Homicide
Murder
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What if someone in Greenland sniped an US Marine in the head and kills him, what happens?
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