A spherical drop, placed in a second liquid of the same density and viscosity, is subjected to shear between parallel walls. The subsequent flow is investigated numerically with a volume-of-fluid continuous-surface-force algorithm. Inertially driven breakup is examined. The critical Reynolds numbers are examined for capillary numbers in the range where the drop does not break up in Stokes flow. It is found that the effect of inertia is to rotate the drop toward the vertical direction, with a mechanism analogous to aerodynamic lift, and the drop then experiences higher shear, which pulls the drop apart horizontally. The balance of inertial stress with capillary stress shows that the critical Reynolds number scales inversely proportional to the capillary number, and this is confirmed with full numerical simulations. Drops exhibit self-similar damped oscillations towards equilibrium analogous to a one-dimensional mass-spring system. The stationary drop configurations near critical conditions approach an inviscid limit, independent of the microphysical flow- and fluid-parameters.