The diseases such as smallpox were brought to the Americas by Europeans and it helped them to conquered Americas. They didn't had to use the weapons because the diseases which were common among Europeans wrecked havoc among the natives. The native Americans have been isolated for too long and never came in contact diseases like small pox, hence they never developed resistance against it and when they came in contact with these diseases for the first time, they started to die in large numbers.
Diseases, Because the weapon that the Native Americans had no resistance to that killed THE MOST Natives were the new diseases that were introduced. Yes the diseases weren't new to the Europeans because they had built up immunity. but the Natives had no such immunity and it killed them like wildfire.
