[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."]