If The United States were to be invaded, how would it be done?

First, they would have about a dozen or so carrier ships enter the atmosphere and hold position over strategic cities say, New York City, Washington, DC, and Los Angeles. Then, they would synchronize their attack with a coded signal that counts down until the moment when each carrier ship's directed energy weapons destroy large swaths of the cities (I'm talking, like 20-50 square mile destruction zones). After the first strike, the carriers will simultaneously move to other strategic cities. The carrier ships will need a protective force field, because the US will likely attempt a nuclear strike after learning the attacking military is hostile. Finally, the only way the attacking military can hope for successful destruction of the US is to mitigate the threat of wiley drunken cropduster pilots that are handed F-18s as a last attempt to defend the country. edit: No such thing as a dustcropper, unless they're in 1930's Oklahoma.