In the 18th century, the British fought France and its Indian allies for possession of what was to become Canada during the French and Indian Wars (1754-63). At the time of the Pontiac rebellion in 1763, Sir Jeffrey Amherst, the Commander-in-Chief of the British forces in North America, wrote to Colonel Henry Bouquet: “Could it not be contrived to send smallpox among these disaffected tribes of Indians? We must use every stratagem in our power to reduce them.” The colonel replied: “I will try to inoculate the [Native American tribe] with some blankets that may fall in their hands, and take care not to get the disease myself.” Smallpox decimated the Native Americans, who had never been exposed to the disease before and had no immunity.
It has been alleged that smallpox was also used as a weapon during the American Revolutionary War (1775-83). During the winter of 1775-76, American forces were attempting to free Quebec from British control. After capturing Montreal, it looked as if they might succeed. But in December 1775, the British fort commander reportedly had civilians immunised against the disease and then deliberately sent out to infect the American troops. A few weeks later a major smallpox epidemic broke out in the American ranks, affecting about half of the 10,000 soldiers. They retreated in chaos after burying their dead in mass graves.
A good short essay on the impact during the War of Independence is the Common-place review of Pox Americana.
Offhand NewMexiKen knows of no other instances in American history where smallpox was used as a weapon. The disease did enough damage on its own. (My own step-grandmother survived it as a child.)