Gene narrates in Chapter 2 of A Separate Peace that he and the other boys have spent the summer of 1942 in "complete selfishness" as they have ignored what was happening in the European theatre of war. Then in Chapter 3, after Finny expresses his disgust with the innocuous, soft games finds a medicine ball and declares, "All you really need is a round ball," as he urges the other boys to get "a little exercise now." Further, he suggests that they create a game, and Bobby Zane suggests that the game have something to do with the war such as blitzkrieg, a sudden, overwhelming attack--a tactic used by the Germans. Having heard Bobby, Finny tosses the word around and decides to create a game called "blitzball." In this game each player is the enemy of all the others; so, when someone is vulnerable, everyone surrounds him and knocks him down.
Gene reflects that no one can play Blitzball as it played by Phineas, whose athleticism is so superior to that of others:
...he created reverses and deceptions and acts of sheer mass hypnotism which were so extraordinary that they surprised even him; after some of these plays I would notice him chuckling quietly to himself.
Finny has created his own blitzkrieg, his own sudden attack in "complete selfishness" upon the realities of war, halting it at Devon School where there is "a separate peace" in the summer of 1942.