Cavalry Refusal

In 1936 it was clear to everyone that if Britain would only help the Spanish Government, even to the extent of a few million pounds' worth of arms, Franco would collapse and German strategy would be severely dislocated... Yet in the most mean, cowardly, hypocritical way the British ruling class did everything they could to hand Spain over to Franco and the Nazis.

—George Orwell, Looking Back on the Spanish War

Our heroes are in trouble. They're being completely slaughtered by the Big Bad's forces, who are obviously too strong for them... but wait! There is a large regiment of cavalry in the area. Let's just call in these guys and the Big Bad will be easily defeated.

Or so you'd think. In fact, the cavalry don't feel like rushing in to save the day at all, and stubbornly refuse your desperate call for help. Perhaps they have some kind of Obstructive Code of Conduct forbidding them to intervene. Perhaps the Big Bad has powerful friends whom they don't want to get into trouble with. Or the Cavalry are organised more or less democratically and some of them are opposed to helping the heroes (or they all agree about that, but argue endlessly about the details). Whatever the reason, the Cavalry refuses.

This trope is in play when the heroes need help, and there is an entity which could and/or should provide it but says 'Screw you'. Can lead to the heroes (and, if the unwilling Cavalry's excuse is particularly poor, the audience) exclaiming 'Why don't you do something?' or 'What The Hell, Hero?' If the heroes looked up to their supposed protectors, this will result in a Broken Pedestal.

Comes in two varieties:

Type A is when the unwilling Cavalry is a (supposedly) neutral authority figure with power over both the heroes and the villains. They don't rush in to protect the heroes, even if the villain is clearly violating the rules which they are supposed to enforce - for example, in a schoolyard setting, a teacher not doing anything to protect the bullied protagonist. Related to Adults Are Useless and Police Are Useless.
Type B is when the Cavalry have no formal power over the villains, but are big and strong enough to significantly aid the heroes. They are supposed to be on the heroes' side (thus making them potential Cavalry), but refuse to help.

Compare Dying Like Animals (when everyone acts this way toward the heroes) and Cavalry Betrayal (when the Cavalry does show up, but turns out not to be on the heroes' side after all).

Examples of Cavalry Refusal include:

Type A

Anime

In Last Exile, the Guild (an organisation supposed to enforce The Laws and Customs of War) does nothing to help an army which is being attacked with "forbidden" tactics, because they favour the offending army.

Real Life

It's hard to keep count of the times the United Nations have refused the Cavalry call, due to the decision-making process which allows several powerful nations, whose interests are often diametrically opposed, a veto.
The United Nations' predecessor, the League of Nations, wasn't exactly eager Cavalry, either - they stood and watched while the Japanese invaded (Chinese) Manchuria, the Italians brutally conquered Abyssinia and a handful of Spanish generals staged a coup against their government which led to the Spanish Civil War.

Type B

Film

In Zulu, a force of actual cavalry arrives upon the scene where the heroes are making a Last Stand... and promptly flees. This happened in Real Life, too, but in reality, the cavalry were much more justified in their flight than depicted in the film.

Television

Supernatural's first order of business after inventing a cavalry was inventing reasons for that cavalry not to help. The angels as a faction are stuck in a revolving door between this and Cavalry Betrayal. Refusal to answer the call happens regularly, but here are some notable examples:
- Castiel turns down Dean's pleas in Season 4's The Monster at the End of this Book, although he does offer a creative solution.
- The Trickster refuses to help the Winchesters when they come calling on the eve of Armageddon. It doesn't take.

Video Games

In the introduction video to StarCraft: Brood War, a Terran colony finds itself the target of a literal Zerg Rush; a Terran warship shows up overhead, but promptly leaves again without doing anything for the besieged Terrans on the ground.
Shepard can pull this on the Citadel Council at the end of Mass Effect.

Real Life

Britain and France in the Spanish Civil War (see the page quote). Eager to appease Nazi Germany and prevent (or rather, forestall) World War II, they took this trope Up to Eleven by enforcing an arms embargo against the Republicans, on whose side they were supposed to be.
Poland was on the wrong end of an indirect cavalry refusal where Allied air raids were prevented by Russia's refusal to let the planes refuel in their territory.
As depicted in both the book and the Michael Bay movie 13 Hours: The Secret Soldiers of Benghazi, the September 2012 terrorist assault on the U.S. diplomatic annex in Benghazi could have been stopped hours earlier had the U.S. government authorized airstrikes or even just flybys from fighter jets over the city. Instead the American personnel on the ground had to fend for themselves, and four people were killed, one of them J. Christopher Stevens, the U.S. Ambassador to Libya.