I've only watched the first couple of episodes of "The Walking Dead" and have not read the comics. It seems to me that they are glossing over what caused the zombies intentionally, or how things fell apart. The attitude being that they are using the lack of knowlege of the characters involved to justify not building up any kind of inherant logic with the series.
That said, how easily people could get overrun depends on the details. If your dealing with a "patient zero" scenario where one infected guy stated it by biting people, who in turn have to bite more people, then it doesn't make much sense.
On the other hand if your dealing with some kind of contagion that was airborne, affected the majority of people just by them breathing. In this case the survivors would be people who were immune to the airborn version, but can be infected by the direct transfer of bodily fluids (ie saliva entering the blood stream), a more concentrated dose of the contagion overcoming their immunity or whatever.
If you say had like 80% or so of the population suddenly drop dead, and then get up and start killing/infecting the remaining 20% by total surprise, the whole situation changes. Those military bases are going to have 80% of their personell turn into zombies, and be inside the bases and so on.
Another possibility is the "slow burn" which is the idea that people don't zombify immediatly, and while a lot of people are infected, they don't start out as a majority like the above. Rather there is enough of them that they overflow the hospitals, and you start seeing other emergency facilities like fire departments, police stations, school gyms, and military bases being used to house the patients. When they finally start to turn, you have the "zombie surprise" starting right there inside those facilities, meaning that the emergency services are going to be the first things overrun in most places.
Then of course there is always the possibility that we're not dealing with a cause rooted in science fiction. Right now the popular concept of a zombie involves them being created by a virus, or perhaps a radioactive mutation. If they are created by magic, or some kind of curse, it might be entirely differant. You know, a scenario where this is the result after the bad ending of a horror movie, and some imprisoned evil escaping or whatever.
At any rate, if you assume the military command structure took a hit during the outbreak, logistics are going to be in rough shape. As Rommel could have told you, all the tanks in the world won't help if they run out of gas. The military relies on a lot of support personel to keep the guys in the field fighting.
There is also naive idiocy to consider here as well. If you assume when the military responds that there are uninfected people being run down and such, people getting out of tanks and the like to try and lend a hand is possible. What's more there are a lot of people who are going to be wary about carpet bombing and the like. By the time the guys in command get their heads around the scope of the problem, it might be too late. It's also possible that you could see the guys in charge (especially if left wingers) wringing their hands, waiting for a magical solution to arise, and then getting overrun because they won't pull
the trigger so to speak.
In a purely pragmatic sense, there is going to be a finite number of Zombies. The logical thing to do would be to write off survivors, aside from those that can be easily collected, and pretty much cut loose without any signifigant care about collateral damage. Not nice, but humanity wins assuming there was enough of a surviving infrastructure. On the other hand emotion does overcome logic in many cases where it probably shouldn't. Indeed the guy calling for this kind of action might be right, but typically would be presented as a bad guy in most dramas.
The point I'm getting at here is there are ways how the scenarios could wind up like what we see on TV and movies, and read in books. Generally speaking though there is usually little effort spent trying to justify it. Indeed it seems 90% of the time when they do try and explain what happened it makes absolutly no sense.
Take for example your typical "quarantined town" example, where the military response seems to be to put a couple of hummers or jeeps on the roads into and out of town. While many would call it evil, if I had that kind of knowlege, I wouldn't be quarantining the town, I'd wipe the whole bloody thing out to prevent the spread. A town full of innocents vs. humanity. When someone in a movie makes the point I am, they are usually portrayed as being the bad guy, on the other hand while nobody wants to be on the receiving end of that, looking at something like "The Walking Dead" pretty much demonstrates that there is no "good" solution, just the lesser of two evils.
