Which branch responds first in an alien invasion?

Long story short I’m writing a literary rpg story that’s about an inter dimensional species opening up portals into our world and releasing thousands of grunts throughout cities across the world. These alien grunts harvest the souls of sentient species they kill, can flip cars and are resistant to regular rounds but can eventually be killed with regular gunfire. In the case of the American response, if every major city in every state simultaneously had to deal with a land assault of several t...