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Setting is absolutely key to this great and rather shocking short story by Graham Greene. Let us remember the context: the story is based in London, and set nine years after the end of World War II. London, much more than other European cities, had been regularly blitzed or bombed by German planes which destroyed large tracts of buildings, making London resemble a bomb site. The rebuild took many years to carry out, so for a long time after the end of the war, the people of London still lived in rubble.
Many were concerned about the moral destruction of society rather than the physical destruction however. An entire generation of children had grown up knowing nothing but war and destruction, and the collapse of hope. This is the kind of gang that we are presented with in the form of T. and Blackie. Note how the setting is described in the story:
The gang met every morning in an impromptu car-park, the site of the last bomb of the first blitz... On one side of the car-park leaned the first occupied house, number 3, of the shattered Northwood Terrace--literally leaned, for it had suffered form the blast of the bomb and the side walls were supported on wooden struts. A smaller bomb and some incendiaries had fallen beyond, so that the house stuck up like a jagged tooth and carried on the further wall relics of its neighbour, a dado, the remains of a fireplace.
Thus we can see that the place where the gang meets already mirrors the internal "destruction" that has gone on within the boys in terms of their values and morals. Bomb blasts have demolished the buildings, and number 3 is poorly supported and isolated. In the same way the boys engage themselves in acts of vandalism and destruction without a care for concepts such as "right" or "wrong."