How did the individuals, events and setting in chapter 2 help to show that John Snows theory.

Using the atomic bomb on the Japanese in WW2 brought the war in the Pacific to an abrupt end. Even after they experienced the devastation of Hiroshima, there was no word from the military leaders that they were ready to surrender. Three days later, the Americans added Nagasaki to the two cities that were destroyed by a single weapon. That brought the immediate surrender of Japan. These two facts will be used to show  

that the world had been brought into a new era of warfare.  Perhaps.

There is no reason why you need just one sentence as a thesis. And what you wrote, to be truthful, is as good if not better than what I wrote. It's shorter.