It is a locution that is used to "come out in defense", always of something or someone. It has its origin in medieval jousting, in tournaments where two knights faced each other with spears and on horseback in order to knock down the opponent. When it was done to save honor from an affront or to end a dispute, it was called "spear-breaking," and it could also be done on behalf of someone who was unfit to fight. Hence the current meaning, which can be extended to "face difficulties to achieve an end". See "Do not break spears with anyone".
