Unless that civilization happens to be Marxist :-)
Don't mind my musings. It helps me to think (I think). You could just as correctly say when governments clash, right? As was the case in the Cold War. Not exactly two "civilizations", but merely two governments. As Patrick Henry (the freeper) said, property is a concept, therefore, it exists through law, which is an instrument of government, yes?
So, in summation, property is not a natural right, as is, say, self-defense. Right?
No, I wouldn't say that. If two nations, albeit governed by different governments, have compatible systems of rights, then they develop a common system of property rights without initiating force. They may resort to a "gentlemen's war" on occasion as part of competitive acquisition of property, but that violence would be secondary to a contractual arrangment, like a boxing match.
If two civilizations have mutually exclusive systems of rights, as, in the Cold War, Western classical system of individual rights versus the communist rights as grants of permission from a central authority, then they cannot coexist without violence and any property dispute between them would often have to resort to force.