This is classic set-up for a scene that - one assumes - will be much more interesting than the set-up. So why not begin with the actual scene instead of beginning with the story line trying to arrive at the beginning of the story? In this case it would mean beginning the moment the horsemen stumble upon the bodies of defeated comrades on pages 2-3.
First thing said:
"We're too late."
On the first couple pages there isn't really much happening. Back story and establishing setting (which is well done) consumes most of the space and there is little glimmer of a forward narrative until page 3. There is some character development, but nothing that stands out and says: I want to read about this person for the next several days.