Homage to the French Armed Forces, and in particular, the Aéronautique Militaire (Air Force).
The Aéronautique Militaire used equipment never yet seen in the short history of manned flight; their planes flew pretty much without vertical stabilisers (tails), and their daredevil pilots stood up in the cockpit and steered (and I mean steered) with one hand! The flag in the other hand was, of course, mandatory. Beware, you Germans!
Not only is this card a good early example, produced at the very start of the war, it was also written and posted not long after, on 23rd September 1914. And tell me this, if you will. What is it about horses that makes George Stubbs the only bloke in the history of Art, or possibly the human race, who could draw or paint one properly?