He knows the men aren't strong enough to open the door.
Explanation:
In Book IX of "The Odyssey" by Homer, Odysseus is on the island of the cyclops Polyphemus, the son of Poseidon. Initially he and his men did not realize it was the cave of the Cyclops so on seeing the abundant food and drinks available, they feasted but was trapped by Polyphemus before they could go back to their ship. Polyphemus had used a big block of stone to seal the entrance of the cave, thereby rendering the mortals unable to escape. Odysseus could have easily killed him but he did not as he knows he along with his men alone aren't capable of moving the stone block that is used as the door. Besides, some of his men were already eaten by Polyphemus so he should use his wits instead of his strength in order to escape from the cave alive.
