Was It? That's a rather parochial way of looking at history. Even if it was theirs did they manage to hold on to it and repel the interlopers? Which particular nations are you referring to? Did Alexander the Great, the Persians, Greeks, Romans, Saxons, Huns, Mongols, Goths, Ottomans, English and European colonizers, etc., take land that wasn't theirs? What about the cannibals of Africa, Central America, and South America? The Spanish Conquistadors with 1500 defeated one million Aztecs. Pizarro defeated the Incas without a single casualty in his force. What is your point or are you just being flippant based on your smiling icon?

Yeah, the natives were aggressive alright, they were fighting each other long before the Europeans showed up.
Fighting over land and natural resources.
1200+ different tribes, some were allies, some hereditary enemies,
some were nomadic, others stayed in place, some were agrarians,
some were h****rs, some were fishers,
they had no central government or unified nation,
half of them didn't even know the other half existed and vice versa.
Many tribes were brutal, they plundered, raped, tortured, k**led and ens***ed members of other tribes.
The Comanches were the worst, they were more vicious with captives of other tribes
than they were with captive w****s. Skinned them alive, cut open the bellies of pregnant women, fed the babies to their dogs, stuff like that. Yeah, those natives, man, they were peace lovin folks fer shur.