>>2
Uh....
No. The Europeans while very very good Indian killers they were did not get rid of *all* the Indians (though they really have disease to thank for that, as you correctly said). America has practiced its own genocide/supression of the Indians. Who do you think Custer and Buffalo Soldiers and all those fun calvary dudes were fighting? Ever heard of the "Trail of Tears?" After the Civil War and Mexican American war, we basically entered into a series of Indian Wars against various Indian Nations. We basically stomped over two other (other meaning non-white) races of people to get this land, and exploited all the rest to acheive our modern supremacy.
As for
>>1's question on why, we did it because it was necessary to grow as a country. That's it. It's the same reason we're in Iraq, the same reason we've propped up various puppet dictators, the same reason we started the Mexican-American War AND the Spanish-American War. Pure realpolitik. We convinced ourselves that we deserved the entirety of the North American continent for not much more reason than "It's our destiny." And we weren't going to let anyone stand in our way. And truthfully, we were not going to be the powerhouse we are if we didn't.