Things look bleak: the Big Bad is on the verge of conquering the world, and the heroes have gathered for one last stand. Just when it looks like all hope is lost, a horn sounds in the distance, and the cavalry arrive to save the day. Whether a ragtag bunch of minor characters, an army of unlikely heroes, or the ultra-heroic Eagle Squadron, the timely reinforcements use their overwhelming force to crush the villains and save the day.
When done right, this trope can be one of the defining moments of greatness of the entire work. When done wrong, however, it becomes little more than a Deus Ex Machina of the most unsatisfying kind. How, then, can this moment be done right?
As with any Deus Ex, one of the keys is to adequately foreshadow the end. This often takes the form of Gondor Calls For Aid, when the heroes petition the cavalry for assistance before going into battle. To make things interesting, the relationship between the two parties is often complicated and ambiguous, making it doubtable that the cavalry will actually show up.
However, I think it goes deeper than this. In order for the arrival of the cavalry to be satisfying, it needs to not invalidate everything that the heroes have already gone through. If the cavalry shows up after the heroes have defeated the Big Bad, and essentially rescue them from a heroic sacrifice, that’s satisfying. If the heroes are still fighting the Big Bad and the cavalry comes out of nowhere to hand them an unearned victory, that’s cheap.
In English 318R, Brandon Sanderson often used the film versions of the Lord of the Rings Trilogy to illustrate this. The Battle of Helm’s Deep was satisfying, because the entire premise was to hold out until the third day. When Eomer arrived with the Rohirrim on the morning of the third day and swept away the Uruk-Hai, that didn’t invalidate King Theoden’s efforts because all he was trying to do was survive.
In the Battle of Minas Tirith, however, Aragorn’s arrival with the unbeatable army of the dead was kind of cheap, because the premise was to defeat the orcs, not to hold out for reinforcements. Gondor could have just stood down and let the orc army capture the city, and they still would have won in the end.
The two genres where you’re most likely to see this trope are westerns (trope namer) and heroic fantasy. Just about every David Gemmell novel involves a cavalry moment of some kind, and I looove it. It’s also quite common in military science fiction, too–basically, any story where war is a major part of the narrative.
The variations on this trope are also quite fascinating. For example:
- Cavalry Betrayal: When the cavalry switch sides in the final battle, utterly crushing all hope of victory.
- The Cavalry Arrives Late: When the heroes defeat the bad guys on their own, and the cavalry cleans up the mess.
- Come with Me If You Want to Live: When the cavalry rescue the heroes from danger midway through the story.
- Cavalry Refusal: When the only ones who could save the heroes refuse to do so.
- Redshirt Army: When the cavalry is utterly useless.
- Bolivian Army Ending: When everything goes terribly, horribly wrong.
