A subject that regularly arises to bedevil space warfare discussion at sfconsim-l is the Lanchester equations. These are a set of differential equations, worked out appropriately enough in the middle of World War One, that quantify a basic military truth: Fortune favors the big battalions.
In fact she favors the big battalions even more than we think. In the movie the bad guys always attack the hero one at a time; even samurai heroes get to finish off six enemy ninja at once before the next six step up to fill a hole. In a real fight, when one battleship has sunk its opponent it simply shifts its fire to the next enemy ship; if the battle is uneven to start with it quickly gets more and more uneven.
The rallying cry at the Battle of Maldon in 991,
Our hearts must grow resolute, our courage more valiant,
our spirits must be greater, though our strength grows less
Is all bleakly to the point in the face of the Lanchester equations: You can fight bravely, but you are going down anyway.
All of which leads, via comments by Doug on my last post, to determinism in history and fiction. Do individuals and their actions count for much, or do the big battalions of historical forces carry the day? In fiction the plucky heroes may not always win but they always matter.
Even in Romance, however, Lanchester has his say, by setting the limits of the plausible. Our hero swordsman may be able to vanquish half a dozen foes because his blade is swift and his heart pure, but we don't push his luck and have two dozen guardsmen descend on him all at once. Or if we do, we let discretion be the better part of valor, and the hero decamps quickly across the rooftops.
Otherwise, the reader begins to suspect that the author is tipping the balance. Of course the reader knows that the author is tipping the balance ... but if the story can't make them ignore it, then the story is in trouble.