Théoden, much like the ents marching to Isengard, is aware that he might die in the battle. Still, he feels duty-bound to go and do good for the world, comforted by the knowledge that even if he and the riders of Rohan are defeated, the kingdom will live on, led by Éowyn. Moreover, Aragorn reminds Éowyn that Saruman isn’t Rohan’s, or Middle-earth’s, gravest threat—Sauron is, he implies.