Can a country defend against invasions?

Question

I know think under "premises/occupiers liability" you aren't allowed to set up things like booby-traps, to stop trespassers / invaders.

Are countries/militaries exempt from such laws?

If so, how are weapon systems like PHALANX CIWS, an autonomous anti-missile weapon system, in use?

Is is a matter of discriminance, in that, a booby trap can harm anything, where as systems such as PHALANX CIWS only defend against, what it thinks are, attacks? Or, is it that militaries follow different laws?

For example, was the shooting down of the Russian jet, by Turkey, illegal?

Answer 1

You could almost define a country as, "an entity that can defend itself against invasions."

Non-sovereign entities are indeed generally prohibited from deploying lethal autonomous defense systems like booby-traps.

But governments and state-like actors, as a matter of practice, choose their own rules. laws-of-war and international-law are not like "regular" law: When it comes down to it, states only follow international conventions and treaties to the extent that they consider it to be in their own interest to do so.

If you start mining your property, you will probably be forced to stop by local law enforcement. If a warlord starts mining his borders, he's going to get away with it until someone with more power convinces or forces him to stop.

Was it "illegal" for Turkey to shoot down a Russian military aircraft? One could cite all sorts of laws and conventions to answer that question. But in practice the consequences of that act are limited to whatever Turkey allows, or to what Russia and its allies can impose on Turkey.