As I write this the Chinese are claiming the South China Sea as their own, Muslims are claiming many areas in Europe as their own and of course claiming Israel as belonging to them, Mexicans are claiming parts of the Southern U.S.A as theirs and so on.  What entitles us to declare land is ours?  That question can be broken in to two parts, since land can belong to an individual and land can belong to a country at the same time.  Lets start with the simplest question which is what makes land belong to an individual?  The immediate thought that comes to mind is if I bought land it's mine.  In reality when we buy land we have to pay taxes on it and if we stop paying taxes the land is taken away from us.  In addition local and federal governments often pass laws that restrict what we can do with "our" land.  Whether we can rent the land or not, how many people we can rent to and so on and so forth is controlled by others.  If others control our land and can seize it if we don't pay them then we can't say that we own it,  at best we only partially own it. 

What if the person we bought the land from murdered the previous owner and took the land without paying for it?  Does the fact that we bought it make it our land?