Deterministic switching of magnetic order parameters is essential for information manipulation in magnetic memory devices. This work introduces the concept of and establishes a unified theoretical framework, which reveals that a necessary condition for deterministic magnetic switching is to break switching symmetries. Such symmetry breaking can be achieved through asymmetric states, barriers, or torques, and this principle applies to both coherent and domain-wall-mediated switching. Our theory provides a symmetry-based interpretation for reported experimental cases of deterministic magnetic switching and a step-by-step design procedure without the need for complicated simulations.