  We investigate the factorization of different momentum modes that appear in
matrix elements for exclusive B meson decays into light energetic particles for
the specific case of B -> pi form factors at large pion recoil. We first
integrate out hard modes with virtualities of order m_b^2 (m_b being the heavy
quark mass), and then hard-collinear modes with virtualities m_b Lambda (Lambda
being the strong interaction scale). The resulting effective theory contains
soft and collinear fields with virtualities Lambda^2. We prove a previously
conjectured factorization formula for B -> pi form factors in the heavy quark
limit to all orders in alpha_s, paying particular attention to `endpoint
singularities' that might have appeared in hard spectator interactions.
