This paper provides a physics-only explanation for the origin of inertia by removing its treatment as a primitive property of matter. It shows that inertial resistance arises necessarily when momentum conservation is enforced locally within finite physical systems operating under finite signal propagation speed. Acceleration is identified as a process of internal momentum redistribution, and inertia as the transient resistance that exists throughout this redistribution. The paper demonstrates that inertia is an unavoidable operational consequence of causality in a relativistic universe, not an unexplained intrinsic attribute.