Actually the Indians were done in by betrayal by the British who had promised to help them and give them supplies. When the U.S. won the Revolution, the British said "fuck you" and packed up all their shit and left. Up until the mid 1790's or so, the Indians were more than winning the war. The American Army couldn't even advance on the Indians without getting slaughtered. The Battle of Fallen Timbers was like the only battle that the Americans won, but that was mainly due to concessions by the Indians on land. In years past, the military would have never made it as far as they did without getting ambushed 100's of time. The Indians had given up so much land they lost the strategical advantage.
If you would like an interesting way to read up on pushing the Indians east, I suggest Alan Eckerts books. He makes up conversations and stuff, but keeps all the important historical context factual.