Jasper's service story unfortunately will probably never be fully revealed in Roller Coaster. There are many reasons for this, but the largest part is that for the last year of his four years of service he was locked in some form of prison and tortured - both psychologically and physically. The end result is that he honestly doesn't remember all of it.
For anyone who wants a bit of his backstory, it's below.
Jasper graduated from high school in 2001 and only a few months after he graduated, the two towers fell. Like a lot of impressionable young men (and women) at the time, he applied to join the army in a matter of days after it happened. He finished basic training before Christmas of 2001.
His first two year stint he was in a small platoon of about eighteen men, among them was Peter, who became a quick friend to him. During that two years, he did more aide missions than anything else though there was also fighting and reconnaissance missions. At the end of those two years, several in the platoon rejoined for another two year tour, but most of them were sent to join other platoons.
Jasper and Peter happened to get placed under the same new platoon which was led by a Sargent named Joham. The platoon had a total of twenty men and three women. Unbeknownst to the majority of the platoon, Joham and one of the soldiers, Nahuel, were secretly insurgents.
Some of the orders that came in Joham scrambled and changed to fit to his needs. So some people who they should have been rescuing they killed and people they were supposed to get reconnaissance on or even kill they sometimes aided. Joham didn't do it with every order, only a few, which kept the platoon from getting suspicious.
About ten months after Jasper joined the platoon, an order was sent because Jasper and Peter were needed to report back in the US because of one of the real reconnaissance missions they went on in the first two years where they'd learned vital information. This is what prompted Joham and Nahuel to change their plans and capture everyone in the platoon.
They used the physical torture and murders of the other soldiers in the platoon to try and psychologically break Peter and Jasper after more traditional techniques like waterboarding, shock treatment, starvation, dehydration, and more failed. Both Peter and Jasper kept their mouths shut because they knew if they spoke they'd die.
Eventually, after almost a year of torture and no sunlight, Nahuel tripped up because he assumed Jasper wasn't as recovered as he was when he unchained him to move him. It allowed Jasper to fight his way free, killing Nahuel in the process. He rescued Peter and they managed to escape from the prison they were in. After getting out, Jasper hot-wired a vehicle - something similar to a jeep.
He drove for well over an hour before the vehicle ran over a landmine. The blast left dozens of pieces of shrapnel in his body and Peter's. A particularly large piece tore into Peter's right leg and by the time someone found them and rescued them Peter's leg was beyond saving. Both Jasper and Peter still have some shrapnel in their body that couldn't be removed due to the risk of the damage removing them could do.
When they got back state side they were told essentially that what happened to them,
didn't, and they were told to keep their mouths shut or else. Jasper went home after about three months of rehab but Peter's rehab was longer. Peter and Jasper lost contact after Jasper returned to Forks.
Approximately four months after he got back was the wedding between Jacob and Bella (his first cousin on his mom's side) which is where the prologue started at.