This verse is the conclusion of the gathering process begun in verse 14.
And they gathered them together to the place which in Hebrew is called HARMAGEDON.
While this appears to be a literal gathering of nations for war, this is not necessarily good military strategy. Thus, the symbolic meaning may be more likely. The symbolic view is supported because John uses the term "like frogs" (verse 13) to indicate that this is unclean/demonic activity. Although, these demons most likely are occupying human beings.
The humans who are gathering, in either view, believe they are doing this upon request of the Antichrist (a jihad, perhaps). They don't realize that God Himself is doing the gathering to fulfill His purpose (Revelation 19:21). According to Revelation 20:4, the millennial reign is coming soon, so the earth must be cleansed of evil. How convenient for God to have had the Antichrist have his followers (marked with 666) assemble in one place.
Geographically, HAR-MAGEDON is a broad valley about 60 miles north of Jerusalem. Literally translated, HARMAGEDON, means the hill where troops are cut to pieces. In the Old Testament, there was fighting there (Judges 5:19-23) and killing there (2 Kings 23:24-29-30). Water is referenced in both passages, while here in Revelation, the Euphrates River has been dried up (verse 12). It is not clear why this distinction.
It is implied the purpose is to make a march upon Jerusalem. The Antichrist has already desecrated the temple most likely and now wants to rule the city. He wants something to rule. Hence the city is not destroyed by missiles or bombs but is to be taken by sheer human numbers. This may fulfill the prophesy in Zechariah 14:1-8 with the 'waiting for the Lord' to follow (Zephaniah 3:8-13).