So it's just the fact that I took over two of their neighbours provinces that they're worried, rather than "You have hit the Year X/Taken province Y, you are about to have the entire might of the native americans land on your doorstep".
Nullus Anxietus then, though no doubt crushing them will make someoen *else* annoyed, and crushing *them* will make a couple of others annoyed... *sigh* I forsee a domino effect of me needing to take over all the Natives. I may need another army...