There is documentary evidence that spreading smallpox intentionally to the Indians was at minimum discussed and probably carried out.
http://www.nativeweb.org/pages/legal/amherst/lord_jeff.html
However, this was during the French and Indian War and was under the British Army, not the American Army.
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We need a definitive analysis by a historian we can trust, rather than conflicting claims that (1) there is “documentary evidence” and (2) there is “no hard evidence.”
If you go to the link, there’s original evidence of possible intentional attempts to spread smallpox to the Indians during the French and Indian War. But by British troops, not by Americans.
As far as a dispassionate account of the facts, I found a neat article.
It discusses the Ward Churchill claims of intentional genocide against the Mandan and other tribes in 1837, and blows it clear out of the water.
It turns out Congress actually had put a program in place to vaccinate Indians. Though probably inadequate, it certainly doesn’t indicate genocidal intent.
In fact, Jefferson provided vaccine for Lewis and Clark to vaccinate Indians as they traveled across the continent. Unfortunately, their vaccine spoiled and became unusable, but again it shows intent to protect the Indians against disease, not wipe them out.
Didn’t Ward Churchill fabricate some of his so-called history to place blame on Americans? Was he not fired from his Colorado teaching job for it?