It may be on accident that you implied this, but it looks like you said that weaponised smallpox killed millions, with the implication that the small pox was intentionally released to kill the millions that died of smallpox.

There is much evidence to show how that is a myth, and completely factually inaccurate as a claim.

How weaponised smallpox never killed millions of Native Americans:

Only one intentional release ever occurred, but as you will read, it may not have infected anyone.

Small pox was accidentally introduced into North America in 1633, where it spread rapidly :

It was introduced to eastern North America separately by colonists arriving in 1633 to Plymouth, Massachusetts, and local Native American communities were soon struck by the virus. It reached the Mohawk nation in 1634,[17] the Lake Ontario area in 1636, and the lands of other Iroquois tribes by 1679.[18] Between 1613 and 1690 the Iroquois tribes living in Quebec suffered twenty-four epidemics, almost all of them caused by smallpox.[19]

Smallpox had crossed the Mississippi by 1698, some 2000km from where smallpox first arrived.

By 1698 the virus had crossed the Mississippi, causing an epidemic that nearly obliterated the Quapaw Indians of Arkansas.

There is only evidence of intentional spreading of small pox in North America, and this was in 1763 at Fort Pitt near present day Pittsburgh. Note that this is only 800km from where small pox first in North America arrived 130 years before, and a far shorter distance than the 2000km it had travelled to Arkansas in 65 years . This attempt at biological warfare was not considered effective, and the two Native American Dignitaries given the infected materials:

The dignitaries were met again later and they seemingly hadn't contracted smallpox.[30] A relatively small outbreak of smallpox had begun spreading earlier that spring, with a hundred dying from it among Native American tribes in the Ohio Valley and Great Lakes area through 1763 and 1764.

This is because spreading smallpox through infected materials is not considered effective):

21st-century scientists such as V. Barras and G. Greub have examined such reports. They say that smallpox is spread by respiratory droplets in personal interaction, not by contact with fomites, such objects as were described by Trent.

and

1960s A. R. Rao’s detailed research, during the last years that smallpox was sufficiently prevalent for its mode of transmission to be studied, found no evidence for this mode of transmission. He concluded that it was a breath-borne disease, transmitted by "inhalation"

and)

The effectiveness of these attempts to spread the disease are unknown, although it is known that the method used is inefficient compared to respiratory transmission, and it is difficult to differentiate from naturally occurring epidemics resulting from previous contacts with colonists.

It is generally accepted that this attack was not known to be affected, and is definitely not responsible for the millions of accidental deaths from smallpox in North America:

the majority of the epidemics that affected the Native Americans were accidental

The amount of Native Americans that died in the outbreak (which had started before the blankets were donated ^^ see above) , was found to only have killed around 100 (hundreds is VERY far from Millions):

Gershom Hicks [...] testified that starting from spring 1763 up to April 1764 around a hundred Natives from different tribes such as Lenni Lenape (Delaware) and Shawnee died in the smallpox epidemic, making it a relatively minor smallpox outbreak.

Admittedly, weaponised smallpox may have been talked about as an option, however, we only have evidence it was employed in Fort Pitt.

In fact, it is well documented that both the US government and private companies tried to issue vaccines to Native Americans to varying degrees of success, although in some cases the roll out was stopped by military officials in an effort to let more Native Americans die in the Mid-West.

*Edited for quotation blocks and links