first action on
I heard this in a British movie.
Just after an assault on a military base, two soldiers get into an argument. Major Henry stops it:
Henry: Mitchell, first action on.
Mitchell: Resecure the perimeter.
Henry: Then get to it. Jones, Bedford, go with him.
Could you please explain to me Henry's "first action on"? The adverb "on" here makes the least sense to me. And what does the full sentence look like (if there were any words left out)?