[Because the attempt would be futile, and would expose the blocking force itself to serious risks. There are two interpretations available here. I follow that of Chang Yu. The other is indicated in Ts’ao Kung’s brief note: "Draw closer together"—i.e., see that a portion of your own army is not cut off.] On ground of intersecting highways, join hands with your allies. [Or perhaps, "form alliances with neighbouring states."]