I feel this song relates to war from a soldiers perspective. I know it was written for Iron man 3, but I personally feel this song has something more to it.

"With our backs to the wall, darkness will fall" from a soldiers point of view could mean that although they are pinned down, they will keep fighting so the "darkness will fall" the darkness being the enemy.
"An empire falls in just one day, you close your eyes and the glory fades"
Shows that although there has been a victory and there is the initial relief of being alive, that soon wears off and you think back on it and realise what you have done to gain that victory, and who got hurt because of your actions, and how it changed you as a person.
The way "Ready, Aim, Fire" is sung, it sounds almost desperate, like there is one chance left, and it sounds almost resigned to the fact this could be the last shot you get to make.

"We don't have a choice to stay
We'd rather die than do it your way"
Sounds like a soldier who has been badly wounded and captured and doesn't have a chance at living but would rather be gone than captured.

"How come I've never seen your face 'round here?
I know every single face 'round here
A man on a mission, changing the vision
I was never welcome here." Makes it sound like the point of view of a sniper who is scanning the town and seeing someone he hasn't seen before, who could be a threat.

"Back in the casing, shaking and pacing
This is the tunnel's light
Blood in the writing, stuck in the fighting
Look through the rifle's sight"
Also supports the theory that it is about battle as "shaking and pacing" could be from battle nerves, and the lines "Blood in the writing, stuck in the fighting
Look through the rifle's sight" makes it sound like a sniper has been pinned and his team is dying and he is trying to make shots.

This whole theme also relates to Iron Man 3, as Tony is desperate to stay alive for Pepper, but the arc reactor is failing him so he is pinned down but still desperately trying to save everyone. He starts to make reckless decisions but still only wants the best.