What was Knowles purpose in writing A Separate Peace?

This question is very broad - to what extent is it
possible to understand all the many different motivations as to why an author chooses to
write a book? However, certainly one of the major reasons Knowles chose to write this
book was to explore the rivalry that can lie at the heart of so many friendships and can
potentially destroy it.

Key to understanding how the theme
of rivalry is explored is that the story is told using first person narration, and
therefore we see everything from Gene's point of view. It is he who assumes there is a
massive rivalry between him and Finny, but as he discovers, it only comes from him, and
not from Finny at all. Confronting this truth and the envy within himself is what
prompts him to knock Finny off the tree. If you want some examples though, you don't
need to go very far into the book. Consider this example from the first Chapter, once
the older Gene in his flashback returns to when he was at
Devon:

The
tree was tremendous, an irate, steely black steeple beside the river. I was damned if
I'd climb it. The hell with it. No one but Phineas could think up such a crazy
idea.

He of course saw nothing the slightest bit
intimidating about it. He wouldn't, or wouldn't admit it if he did. Not
Phineas.

Note here how the
attitudes of Finny and Gene are compared. Gene, naturally more cautious and
safety-aware, sees the tree and personifies it with the emotion "irate" (obviously
adding to the danger element) and then uses a metaphor to compare it to a black metal
steeple. He reflects his determination not to climb it very strongly and seems to
disparage Finny for thinking up this crazy idea. However, from Gene's perspective, Finny
is completely unphased by this. Note the use of "of course" to reinforce this
impression. Also note the doubt that Gene instills about his friend with the inclusion
of "wouldn't admit it if he did." The final two words, "Not Phineas", seems to round
off Gene's assessment of Finny as a fun loving, danger seeking character who always
appears to be fine with situations even if he has to hide his fear within him. Such
dialogue and narration highlights the differences between them and the envy that Gene
feels at those differences.

Of course, at the end of the
story, Gene comes to the tragic realisation that he, like everyone else in the story
except for Finny, "constructed at infinite cost to themselves these Maginot Lines
against the enemy they thought they saw across the frontier", only to find that there
was no enemy at all.