If aliens invaded how would the military react?

I mean I know our military has technology that they wont show us and have way more advanced stuff than whats on the field right now but lets say these "aliens" invaded all major cities and started killing everything in its sites and the worlds militaries would have to co operate with each other how would the US Military and the National Guard combat these threats and get all non military personnel out of the city and would they use nukes and fire them at will? Also since they obviously have greater technology and weaponry's than us how would they combat them we obviously do have numbers on our side theres almost 7 billions of us