Following hints from David Aune, Alastair Campbell argues (in an Evangelical Quarterly article) that Revelation 19:11-21 does not describe a last battle but a triumphal parade modeled on a Roman Triumph.
He quotes the following description of such an event: “The victorious general whom the senate had granted the right to a triumph, entered Rome standing on a high two-wheeled chariot, the currus triumphalis, which was drawn by four horses… The triumphator is clothed in the vestis triumphalis: the tunica palmata – thus called after the palm branches embroidered on it – and the toga picta, a name it owed to its rich embroidery, according to Appian in the form of gold stars. Both garments were purple, and there is reason to suppose that originally the toga was purple all over and that the gold-coloured ornaments were a later addition. On his head the triumphator, and his military suite, wore the corona laurea, the symbol of triumph, and for this reason often called the corona triumphalis . . . In his right hand the triumphator carries a laurel branch, in his left an ivory sceptre surmounted by an eagle: in addition he wore the bulla, whilst his face was, in ancient times at any rate, red-leaded’ (6-7).
Just so Jesus, the triumphant world Emperor: “Revelation depicts a procession led by a figure on a white horse, the symbol of victory, followed by his victorious, white-clad, soldiers, also riding white horses. The fact that he is mounted and not in a chariot serves to bring out his oneness with those who, like him, have borne faithful witness. All are riding white horses. The rider is already crowned, as a trimphator should be. Like his Roman counterpart John’s rider is dressed in a purple robe, but with the significant difference that the rider’s robe owes its colour to its having been dipped in blood” (7).
This fits neatly into the chapter, which follows the overthrow of Babylon and the lament over her: “First John hears the jubilant crowd celebrating the victory of the Lamb (19:1-8). Then the conqueror himself comes into view at the head of his victorious army. As we have seen, he is crowned already, because the battle is over, so he cannot possibly be riding out to war. His white robe likewise bears the marks of battles already fought and won. The birds of the air that are summoned by the angel to eat the flesh of the defeated armies, while offensive to modern sensibilities, were a common sight on any ancient battle-field on the day after the battle. Finally, as with a Roman triumph, we see the defeated enemy paraded behind the victor, and the scene ends as Roman triumphs did with the execution of the most prominent captives.The kings of the earth do not then and there confront the rider and his army. Rather, they are paraded before John’s gaze as those who had been gathered together to make war against the rider on the horse and his army and are now defeated. Similarly, the beast and the false prophet have been made prisoners already and are now taken away for execution” (8).
(Campbell, “Triumph and Delay: The Interpretation of Revelation 19:11-20:10,” Evangelical Quarterly 80:1 : 3-12.)