Breakup of planar liquid sheet sandwiched between two moving gas streams has been analyzed using a perturbation method and retaining terms up to second order. The analysis modifies similar solutions found in literature and investigates the impact of the modification on the final results. The effect of gas velocity (both higher and lower than the liquid velocity) has been investigated in detail for a wide range of parameters. Both sinuous and varicose modes of disturbance have been considered. Based on the maximum growth rate obtained from linear analysis and the breakup length from nonlinear analysis, both sinuous and varicose modes are found to be the destabilizing modes for different parametric ranges. The predictions are found to be in excellent agreement with experimental observations reported in literature.