When a country, a group, or an individual which might have been expected to tamely accept the theft of its possessions or other bullying fights back, and thus sends a message that it, they, him, or her, is not to be trifled (or f--cked) with by the bully or by anyone else.
Similar to the Stonewall Effect (q.v.)
After the U.K. retook the Falklands, the Argentines never attempted another invasion, and there was a noticeable drop-off in other types of adventurism by tinpot despots around the world, largely because they had been chastened by the Falklands Effect.
When members of a marginalized community with a reputation for not defending itself actually defend themselves in a way that sparks not only immediate resistance, but also a long-term movement for equality. Derived from the Stonewall Riots of June, 1969, which are generally regarded as a turning point in the history of LGBT civil rights activism throughout the world.
When the kids from the Glee Club gave the guys from the football team a sound thrashing, it triggered a stonewall effect throughout the school, and nothing was ever the same again.