I wouldn't be surprised if the clean shaven soldier is some kind of marketing conspiracy.

People have shaved for all of history, but the idea that the default is a clear chin is something that only started during WW2 and with the following GI Joe comics. Beards go in and out of style, but to consider the clean face an ideal probably comes from the American religiously puritanical way of thinking.

The image is part of the cultural spread of the American Dream and sold alongside Gillette, Frosties and Coca Cola through the American "liberators" of Europe.