Gene Forrester, the main character in A Separate Peace, does eventually enlist in the war, but he never leaves the country or sees battle. The story focuses on the summer before his senior year, which is 1942, and rounds out the rest of that year right before he is eligible to get drafted into the war in the summer of 1943. If a boy didn't enlist right after high school, or by the time he was 18, then he would have been drafted. The war was like a deadly light at the end of the tunnel of his senior year. He tries to avoid thinking about it and focus on studying as much as possible, but the war is always in his future.
Most of what the reader discovers about Gene's war experience is at the beginning of chapter 10. Gene explains his experience as follows:
"I went into uniform at the time when our enemies began to recede so fast that there had to be a hurried telescoping of military training plans. Programs scheduled to culminate in two years became outmoded in six months, and crowds of men gathered for them in one place were dispersed to twenty others. . .The closer victory came the faster we were shuttled around America in pursuit of a role to play in a drama which suddenly, underpopulated from the first, now had too many actors. Or so it seemed" (138-39).
Basically, Gene entered the war and got shoveled around for different trainings to prepare his group for war, but they never found a niche for him to fill before the war ended. In the end of the book he says,
"I never killed anybody and I never developed an intense level of hatred for the enemy. Because my war ended before I ever put on a uniform; I was on active duty all my time at school; I killed my enemy there" (204).