In “Old Man at the Bridge,” what does the narrator keep watching for as he visits with the old man?

The setting of "Old Man at the Bridge" is during the Spanish Civil War. The narrator, who is a soldier for the Republican side of the war, finds this lone elderly man sitting near the bride, unable to walk any further. He tells the narrator, "I am seventy-six years old. I have come twelve kilometers now and I think now I can go no further." But the narrator has a growing sense of urgency to clear him from this area because he expects the enemy line to approach at any moment. This urgency is captured here:

I was watching the bridge and the African looking country of the Ebro Delta and wondering how long now it would be before we would see the enemy, and listening all the while for the first noises that would signal that ever mysterious event called contact, and the old man still sat there.

He tries to engage the old man in conversation in order to motivate him, but the man instead is worried about the animals he has left behind. Taking care of these animals is the reason he'd stayed so long in a place that he'd been told was going to take heavy artillery fighting. Near the end, he tries to stand but cannot. The narrator knows that the enemy's approach is imminent and concludes with, "There was nothing to do about him."