The Native Americans are not a civilization? Neither were the greeks then! The natives didn't "advance" in many of the ways as euro-civ's, and were happy with the earth given to them. They may have been reduced to indigenous people who are oppressed, but it wasn't always like that.
They, as seperate tribes and tribal nations secured north america, until foreigners landed on their soil. Unfortunately for them, the foreigners were way more advanced, but the natives did obtain military victories, and recognized treaties (that were dishonored). If the natives would have secured their borders with Mexico and the mississipi river during the civil war, is it possible the natives would still be around today? i wont debate that here. I will simply say to call the native americans a "non-civilization of minor people" is offensive to me.
Would it? Any stronger than USA? They could be set back in tech, I would be content with them being a "smaller" empire like the aztecs, spawning by the rocky mountains, ending about the Mississippi. Then america spawns.
From this point, there are many possibilities of war and peace in the american continent.
Last and not least, if nothing else, independent cities could spring up a certain year to give america a challenge when it wants to expand.