Because of the vast number of commuters, a major problem facing modern megacities is:

Megacities have an impact on public health, like contamination of water. Overpopulated places can bring diseases and a lack of health protection.

Explanation:

Megacities are the base for gas emissions that could be very dangerous for the environment. Traffic jam is getting worse day by day. Almost half of the habitant of one city spends at least one hour in the car to get to work.

the only other powers were germany russia and uk and they were too busy fighting each other wearing each other down   that the us was becoming strong and they were becoming weaker