In the face of invasion, what level of force is unjustified?

None.

That's right: the United States is a sovereign nation with the absolute right to defend its borders with whatever force is necessary to repel and defeat invaders.

The Left wants to shame us into national suicide, like they've done to the Swedes, the Germans, the English, the Canadians, etc. Are you going to let that happen to us?

https://pjmedia.com/michaelwalsh/trumps-merkel-moment-and-ours/