Bucky didn't know much except that they were surrounded and that Steve was-- he wasn't really sure. Some part of him knew this was his boyfriend, but that didn't make sense as to why he was in a war zone and so a superior officer or a comrade in arms was an easy enough substitution to make.

"Sir-- yes, sir."

He made as if to lower his weapon, but then everything went wrong. One of the other teenagers saw he was going to lower the gun and decided to try and be a hero and tackle him. The hit to his legs caused Bucky to fire, the bullet going straight through Steve's shoulder and ricocheting down the tunnel, though thankfully nobody else was hurt.

Staring in horror at what he had done, thinking Steve dead rather than knocked back with a shoulder wound, Bucky took off running down the tunnel to the sound of a hail of bullets at his back.