A cavalry charge was tried once in the opening days of WW1. A machine gun took care of it. Tanks have machine guns.

P.S. Horses where then relegated to hauling equipment around, not combat. This persisted even into WW2. The Germans used horses extensively for transport. You don't see it much in the documentary films, as the cameramen were instructed to not show the horse drawn stuff. They wanted to present the image that the Wehrmacht was mechanized.