里应外合

1.work in collusion, one from without and the other from within; a concerted attack from within and without; act from inside in coordination with forces attacking from outside; act from the inside in coordination with sb. from the outside; act in concert with the attackers outside; attack from within and without; attack the enemy from within in coordination with operations from without; bore from within in league with forces from without; by action from within and without; collaborate from within with forces from without; front attack helped by people inside the city; in attacking from the outside, coordinate with forces on the inside; strike a concerted blow from both inside and outside; strike (together) from without and within; with one's cooperation from within; work together, both inside and outside