[johniegrad]

Only when they pose an eminant threat against the U.S.

Actually, the term is "imminent threat". Nonetheless, can you clarify what constitutes an imminent threat? How would you define it and who would determine it?

[Demidog]

Forces on the Mexican or Canadian border. Forces on either seabord. Forces on the border of Alaska. Forces inbound to Hawaii. Missiles fired from any nation in an attempt to attack the U.S.

That about covers it. Any of those is an act of war and merit a swift and deadly response.

It's easy to figure out if you just imagine what course of action you are allowed to take when you are yourself threatened. When are you as an individual allowed to use deadly force?