Raphael Lemkin's legal definition of genocide requires that it be:

1. State policy.

2. Intentional.

Before 1783 America wasn't a state.

In the late 16th and early 17th centuries most of the extermination of native Americans happened due to the germs component of Guns, Germs and Steel. The English settlers viewed the results as providential divine intervention, but didn't have a hand in it. Hence the lack of intentionality that's essential to Lemkin's definition.

The two conditions of state policy and intentionality were present only during Andrew Jackson's Trail of Tears operation . You could resonably label Wounded Knee as a genocide operation. The legal question is whether the US army intended to eliminate "in whole or in part" the Lakota people.

Under Grant, state policy was to protect Native tribes, a policy resented by settlers.

No US intervention outside of North America (Haiti, Nicaragua, Philippines, Cuba, Guatemala, Chile, Colombia, Vietnam, Iraq, Iran....) had genocidal intent, not even territorial conquest, much less the kind of genocidal demographic change Russia has pursued time and again as state policy. So the two empires are in no way comparable. To say this in no way implies any defense of the often criminal nature of US interventions abroad. But American crimes carry a different legal label.