Things heat up immediately in chapter 21 with the moment when Robert, who is sleeping outside encounters a cavalryman and shoots him. That decision leads to a nerve racking day that culminates with doubts creeping in to Jordan and the end of Sordo's band in the mountains.
Bullet points between pages 253 - 351
* Robert shoots a cavalryman and then has to prepare the group for action
* He becomes in effect the leader of the group
* El Sordo and his forces are wiped out by soldiers and planes
* Robert sends a message through to General Golz trying to get the bridge blowing mission cancelled
* On the eve of the attack Pablo disappears taking some dynamite and the detonator
Where Hemingway differs from some authors is that he is determined to provide different points of view. Some others would stick with Jordan and relay that Sordo had been killed through the sounds of the battle and the discovery afterwards by Pablo of the corpses but he goes much further. So you get to be with Sordo when he dies, in the minds of the officers trying to kill him on the hill and get the responses to the action from Robert and those around him. You even get to be with the messenger he sends to Golz and get to share his feelings about the battle.
All of these view points add to the feeling this is starting to climax in the final day of the battle. Tomorrow should be an eventful day in the book...