Rev. 19:11-16 is one of the classic crux texts for postmillennialism. This is tough, because many premillennialists consider this a classic crux text for their view. They interpret it as referring to the Second Coming of Christ, after which Christ inaugurates his literal, 1,000 year millennium in Rev. 20:1-6.
John’s Apocalypse ends with 7 visions. Each of these begin with the words “and I saw.”
The language here is symbolic rather than literal, as much of Revelation. John is not speaking of the Second Coming here. He is speaking of the conquest of nations by the preaching of the Gospel. This conquest is a covenantal conquest. Jesus conquers the nations as covenant suzerain and makes the world his vassal servants.
This is why the language used is similar to the sort of expressions we see in Christ’s final judgment. It is also possible, and this is my own argument, that the “war” here is the covenant curses being literally poured out upon the unbelieving world, which refuses to believe the Gospel even as its message becomes more prevalent and more clearly proclaimed(cf. Lev. 26, Deut. 28). I would argue that this is a perfectly biblical description of Yahweh’s “parousia”, that is, his presence or his coming. He redeems the elect and punishes the ungodly. This is a long, drawn out historical process which.
That understanding Rev. 19:11-16 is a perfectly reasonable understanding of the text is clear from New Testament teaching elsewhere which speaks of Christ being crowned king of kings and Lord of lords, as he is described here
The New Testament teaches in no uncertain terms that Jesus is now, during the present age of the Gospel, subduing every enemy. He has been crowned King of kings and Lord of lords at His ascension to the right hand of God (Acts 2:34ff; 5:31; Eph. 1:18ff.; Heb. 1:3; 10:12; etc.). All authority in heaven and on earth is already His (Mt. 28:18). Furthermore, the clearest passage in the Bible on the time of the second coming of Christ includes the declaration that Jesus, who is now reigning over God’s creation (Mt. 28:18), “must reign, till he hath put all his enemies under his feet” (1 Cor. 15:25). Paul is quoting Psalm 110:1, the Psalm of the Messiah as Melchizedekian King-Priest, frequently quoted in the New Testament. Jesus’ reign, including the defeat of all enemies, is the logical application of his cross: “but he, when he had offered one sacrifice for sins for ever, sat down on the right hand of God; henceforth expecting till his enemies be made the footstool of his feet” (Heb. 10:12-13). It should not be necessary to point out that “enemy” is the vocabulary of martial symbolism(Smith)