[en] Quantum electrodynamic cascades in intense electromagnetic fields arise when the proper electron acceleration χ, expressed in Compton units, can attain values greater than or on the order of unity. For times t || 1/ω, where ω is the carrier frequency of the field, we have derived a general formula for χ of an initially resting electron in an arbitrary electromagnetic field. Using this formula, we have found an optimal configuration of colliding laser pulses, which provides a significant reduction in the threshold intensity of occurrence of cascades up to a level of ∼10²³ W cm^-2. (extreme laser radiation: physics and fundamental applications)