Any invasion force on land would have to attack either from Mexico or from Canada. I don't see it as remotely probable.

An invasion force with a slightest chance of success would have massive seaborne and airborne components, along with armor, and would start with a massive aerial and missile attack at the area of prospective invasion. This is not something a well-regulated militia with small arms would be effective against. If the current war in Europe is any indication, small arms are not even effective against armed consumer-grade drones.

What a militia might be efficient against could be e.g. short-term incursions by drug cartels, or similar.