Humanity is a big space fairing empire, only rivaled by an alien race and their empire (name is wip). Humanity and this alien empire have been fighting an on and off war for a few decades. Nothing serious, mainly just border skirmishes and muscle flexing from both sides. But humanity made some interesting geological discoveries on planets within their borders. Very ancient pieces of technology; Jump engines, weapon systems, even large ship hulls that have interesting alloys. Deemed only as "Forerunner" since no one has yet to decipher the language found on recovered data caches. 

After a few years the aliens learn of these discoveries and claim to be the rightful owners of the tech and demand it back. But nothing found matches what the aliens have. Frustrated, the aliens launch an offensive on humanity to take it back by force.

It's been well over 4 years since the start of the war. The border between the two has gone back and forth many times. Humanity has made two more discoveries, and is rapidly pushing to find more. This is to expedite the process to learn why the aliens started the war in the first place. To find out what these things hold that is so important.

[hr]
Well, this is a summary (albeit a crappy one). 
But if you want more details or have questions feel free to ask. Because I was quite tired when I made this, so I wouldn't be surprised if I left you confused because I didn't put enough info in.

Oh yeah. Our characters would be in the military. But where in the military can actually be discussed as I feel there can be multiple different ways to approach the story and the conflict.