<p>This paper examines the development of causal perturbation theory, a reformulation of perturbative quantum theory (QFT) starting from a causality condition rather than a time-evolution equation. We situate this program alongside other causality-based reformulations of relativistic quantum theory which flourished in the post-war period, contrasting it in particular with axiomatic QFT. Whereas the axiomatic QFT tradition tried to move beyond the perturbative expansion, causal perturbation theory can be thought of as a foundational investigation of this approximation method itself. Our reconstruction of this forgotten research program offers new perspectives on contemporary debates about relativistic causality conditions and the problem of ultraviolet divergences.</p>