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We meet Agustín when Robert Jordan and Anselmo return from surveying the bridge on the first night. He's on sentry duty, but is not very good at it – he can't remember the password. He tells Robert Jordan to watch his explosives.
Being on sentry duty, Agustín is absent during the confrontation with Pablo over the bridge that night.
The next morning, Agustín arrives at the cave and has a brief chat with Pilar. He says it's important to keep Pablo alive, because they'll need his cunning if they're going to pull off the mission.
Agustín's present in the cave that night. Pablo is irritating beyond belief, and Agustín loses it. He slaps Pablo on the face, and does it again when Pablo calls him "negro."
When Pablo leaves the cave to sulk and the others confer, Agustín is all for killing Pablo. Wasn't he just saying this morning that…? Oh well, apparently he forgot.
When Pablo comes back in, Agustín works himself into a rage, and it looks as if it will turn murderous. Robert Jordan pulls him aside to warn him that having a firefight close to the dynamite in the cave would be capitally dumb. Oh. He then changes tactics and pretends to be Pablo's best friend. It's clearly pretending.
Agustín reacts to Pablo's claim that he's decided to help them all by saying he doesn't trust Pablo. Throughout the rest of the evening he continues to compare their situation to a lunatic asylum.
Next day, Agustín goes with Robert Jordan, Anselmo, and Primitivo to man the machine gun over the pass to the cave – he gets to be the gunner. When a fascist patrol of four approaches, he has to fight the overwhelming urge to "make a massacre." He succeeds, barely.
Later in the day, he and Robert Jordan have lunch, and he confesses to Robert Jordan that he has been in love with Maria. After making sure that Robert Jordan does indeed love her, he pledges himself to serve them in any way he can.
On the last day, Agustín fights with Pilar's group to take the upper sentry post at the mill.
He returns from the post with Primitivo carrying a wounded Fernando, whom he leaves by the bridge to die. After the bridge is blown, he provides gun cover for Pablo with Robert Jordan as Pablo crosses from the other side.
As Agustín retreats with Pablo and Robert Jordan, he guesses that Pablo has killed his recruits and is enraged. But they all have to worry about escaping, so he doesn't get to make a massacre of Pablo.
When Robert Jordan is wounded, Agustín is the last to ride away. He offers to kill Robert Jordan, but Robert Jordan refuses. Agustín leaves with the thought: "What a whore is war."