Technically speaking, I believe the Mexican American war was the last time war was seen in the borders of what is now the US, but Texas may not have been a state or territory yet.

I forgot what decade the MA war happened. Yep Civil war is the last time enemy troops were on our land, large scale. There may have been a very few Japanese sabateurs that got into the mainland US during WWII, but I wouldn’t count them as an invasion.