We study the phase behavior of diblock copolymers in the presence of an external electric field. We employ self-consistent-field theory and treat the relevant Maxwell equation as an additional self-consistent equation. Because we do not treat the electric field perturbatively, we can examine its effects even when its magnitude is large. The electric field couples to the system's morphology only through the difference between the dielectric constants of the two blocks. We find that an external field aligns a body-centered-cubic phase along the (111) direction, reducing its symmetry group to Mm. Transitions between this phase and the disordered or hexagonal phases can occur for external electric fields ranging from a minimum to a maximum value beyond which the R3m phase disappears completely. This electric field range depends on diblock architecture and temperature. We present several cuts through the phase diagram in the space of temperature, architecture, and applied field, including one applicable to a system recently studied.