Afghanistan, and similar theatres to that, are not like modern USA. To occupy and control an area there, you need to go, on foot, farmhouse to farmhouse, in a slow, manual fashion, because people are "off the grid". Much more like what you had to do everywhere when the 2nd amendment was written, ironically enough.
The vast majority of the population today (here) lives not in rural areas, but in dense urban centers. People dont grow their own food and livestock on their own land (for the most part). They are on the grid, and dependently so.
If I were to quarterback an assault against any major city, what would I do?
First target: Utilities: kill the power, phone, internet, and water supply. Some person in their 20th story condo or townhouse on residential street, waiting to shoot back with his AR-15 at the foot soldiers will be waiting a LONG time, cause they aint coming. With no food, water, electricity you'd be out of food pretty damn quick. You could maybe gather at the nearest restaurant or store, but with no means to refrigerate food, that wouldnt last long either.
Control the airport, and fuel distribution centres. Run outta gas and you cant go far, and since nobody owns a horse anymore, you gotta walk or ride your bike. Given obesity rates, mass migrations in a hurry to congregate and defend sounds unlikely.
Before long you got a starving, cold, isolated group of people who find themselves in a nasty spot, compared to the Afgan rebel whose family has not come to be dependant on running water, electricity, cars or internet service in generations. He can milk his cow, eat his eggs, survive with limited shelter just fine, and he probably knows how to make an IED off the top of his head. If the internet wasnt down, your accountant across the street could probably look up how to make one too, but....
Taking those key targets with the aid of apache helicopters wouldnt be too hard initially,the "150 million" armed citizens with their 300 million guns would not be pre-massed there ready to defend. Once you have control of that, if the citizens to organize into small groups to try and take them back, I would bet it would be much easier to defend once established, against small arms fire.
I'm no military strategist, so I'm sure those who have served may be able to poke small holes in my plan, but hopefully you get the idea where I'm going with this.... and no, it doesnt require using F-18's to carpet bomb suburbs and kill millions of people right away.