Clarification: the 100 Million + figure includes both North and South America and dates back to 1492. By the end of the 16th century, the number of native peoples killed on both continents numbered around 60 million. The US didn't become a country until two centuries later.
I'm not justifying it, and certainly not denying it. But the history books are lacking in several areas.
Methods used? Outright slaughter, not just in Battle but upon native home sites where they lived. Disease was another killer, as I understand. Intentional or not, it has to figured in.
While not actual genocide, native populations were also reduced by breeding slaves. An analysis of the genetic material of the Hispanic population will track their DNA signatures back to Spain, Africa, as well as the Aztec, Mayan, Inca, and other tribes. In the Caribbean, you can add in the Irish as well.
With the exception of slave breeding, the Americans used these methods and one more onerous trick: starvation. Besides the Indian agents failing to provide goods that the treaties promised, the native food supplies were decimated for sport. Don't know if they teach this to kids today, but I recall learning about "sportsmen" that would have a train stop so they could have target practice on herds of bison. They left the carcasses in the sun to rot while natives starved.
Good start on teaching a little history. Anyone that wants to learn more, Google Native American Genocide.
Hope this helps.