From Chapter 1 of Bullets and Billets by Bruce Bainsfather

I stood in a queue of Gordons, Seaforths, Worcesters, etc., slowly moving up one, until, finally arriving at the companion (nearly said staircase), I tobogganed down into the hold, and spent what was left of the night dealing out those rations. Having finished, at last, I came to the surface again, and now, as the transport glided along through the dirty waters of the river, and as I gazed at the motley collection of Frenchmen on the various wharves, and saw a variety of soldiery, and a host of other warlike props, I felt acutely that now I was in the war at last - the real thing! For some time, I had been rehearsing in England; but that was over now, and here I was - in the common or garden vernacular - "in the soup".

The author is using what type of figurative language to compare war to a theater in the above passage.