Humans found a portal to another dimension. After deciding to venture through it to see what was on the other side, what they found that made any sense on the other side, was that nothing made sense.
Unfortunately the entire expedition was wiped out by unkown attackers. The attackers came through the portal to earth not long after. Attacking the humans without remorse or warning.
After a long, seemingly unending war with the invaders, after being pushed to the brink, humankind managed to turn the tide of the war, by using the attackers' tech against them.
Wanting to avoid another incursion, humans sent a group of people to the other side to defend the portal.
You are the general sent through to run the defence operation, to try and stop them as long as you can.