Political Cartoon

In this political cartoon, I believe that the cartoonist is trying to say that trench warfare, a major tactic used on the battlefields of the western front, was a lot like submarine warfare.  While in the trenches, one was isolated from the surface world, like a submarine underwater.  The only safe view of the surface would be through a periscope.  I also believe the cartoonist is saying that trench warfare was the deadliest of all.  There was no safe place to be found, and as a result, the western front saw the most loss of life.