It's true that by modern standards the ad is odd. By by wartime WWI standards, it's not that astonishing :
- Ads always aim at inspiring hate of the ennemy (see how cruel that big evil German looks ?);
- We're always winning the war, whatever happens on the front.
Add to that the grip of censorship. And then again, wartime propaganda usually defies common sense. The main message at the onset of WWII was "we will win because we are the strongest".

I think the pen symbolically represents Alsace and Lorraine, the two regions occupied by German for almost half a century at the time, and the main objective for the French.