The Greek uses here the definite article so as to speak of “the battle.” The use of the definite article commonly means that the noun involved had been referred to earlier, in Revelation 16:14 and Revelation 19:19 (both with the definite article), and in Revelation 11:7 (without the definite article). But in the broader scheme of biblical revelation, “the battle” stems back ultimately to the hostility God placed between the seed of the woman and the seed of the serpent in Genesis 3:15, with the promise that ultimately the offspring of the woman would destroy the offspring of the serpent. In the present passage, this offspring is assembled for ultimate crushing.
8 and will come out to deceive the nations that are at the four corners of the earth, Gog and Magog, to gather them for battle; their number is like the sand of the sea.