As a comparison, what were the limiting factors that prevented hostile activity in WW1 and WW2 from spreading? How does modern combat differ? What would prevent hostile activity from reaching certain areas now?

I believe the major differences to be mobility, precision, and production capabilities. We can move more farther and with much greater effect. There is no logistics barrier anywhere anymore. 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.