As a comparison, what were the limiting factors that prevented hostile activity in WW1 and WW2 from spreading?
I don't think there was a limiting factor. However, united, the Allied Forces stopped Germany and Japan. If the Allied Forces did not stop Germany, we may be speaking German now.
How does modern combat differ?
We have nuclear weapons and some have the means to take down missiles.
What would prevent hostile activity from reaching certain areas now?
Israel has a smart, strong defense.
I believe the major differences to be mobility, precision, and production capabilities.
I don't think mobility has changed much since WW II. Though the zombies will have the means of obtaining nuclear weapons, if they haven't already, most of their weapons are either primitive or improvised.
We can move more farther and with much greater effect.
With missiles, we can. We also now have drones.
There is no logistics barrier anywhere anymore.
With the zombies there are but they have their own means of delivering bombs.
The quantity and quality of materials and equipment is far superior, and the means with which to both find and target an enemy effectively is nearly indefensible today.
Yes and no. Zombies are one of the enemies we face, after all. I believe the zombies are the ones who pose the greatest threat to the United States.
Jeanette Isabelle