It's not yours if you can't defend it.
But Kuwait was NOT a legal part of Iraq, when the 1973 Helsinki Final Act was signed that said the borders of countries are FINAL and no changes can be made, except by consent of the sovereign country and those wishing to secede.
On the other hand Kosovo WAS part of Serbia in 1973, when the US and a host of other Western countries signed the Helsinki Final Act.
"possession is nine-tenths of the law".
Not when it is theft.
New York wasn't ours on 9/11 because "we couldn't defend it"? The Pentagon "wasn't ours on 9/11 because we couldn't defend it"?
The rule of force is not the rule of law, and rule of law is what civilized societies embrace, not the rule of the jungle.