A new theory is presented to explain the yield stress anomaly for (111) slip in L1(2), alloys. The screws are assumed to cross-slip from (111) to (010) forming locks lying partially on (111) and (010) or completely on (010) (Kear-Wilsdorf locks). These locks are joined by glissile edge super kinks which are themselves pinned at low temperature by edge dislocation dipoles at the ends of the cross-slipped screws or at high temperatures by Kear-Wilsdorf locks terminated by sessile edge dislocations on (010). The yield stress is controlled by unpinning of the superkinks, which bypass the locked screw dislocations, by mechanisms which are thermally activated but have a larger athermal component. The increase in yield stress with increasing temperature is due to the increasing probability of cross-slip and formation of locks by the screw dislocations, which results in a corresponding decrease in the lengths of the superkinks. The theory accounts for the mechanical properties, including the small strain-rate sensitivity of the yield stress. and explains the electron microscope observations.