Via the US Library of Congress, this fantastic simple British recruiting poster from 1915.
Really fantastic handdrawn typography and an eye for proportions. A classic of the type.
The whole Library of Congress WW1 Poster Collection is fantastic and well worth a browse. WW1 and post-WW1 posters from all over the world, not just the English-speaking world. Even better, copyright has long expired on almost all of these items across most of the world, so you can re-use them for your own purposes if you like.