Here is the verse:

Revelation 19:11 (NASB):

"And I saw heaven opened, and behold, a white horse, and He who sat on it is called Faithful and True, and in righteousness He judges and wages war."

Why it is not a future event:

Audience relevance: The book of Revelation was written to first-century believers (Revelation 1:1-3), and they were told the events were "soon to take place" and "the time is near."

Revelation 19 is about the destruction of Jerusalem (70 AD):

The rider on the white horse (Christ) is symbolically coming in judgment against the apostate harlot (Jerusalem, Rev 17–18). His waging war in righteousness matches Matthew 22:7 –

"But the king was enraged, and he sent his armies and destroyed those murderers and set their city on fire."

Compare Revelation 19 with Matthew 24:

In Matthew 24:30, Jesus says they would see the Son of Man coming on the clouds of heaven with power and great glory in that generation (Matthew 24:34). Revelation 19 is a symbolic depiction of that same event.

He already rules with a rod of iron:

Revelation 19:15 says "He will rule them with a rod of iron" – but Psalm 2 and Revelation 2:27 show this is already fulfilled in Christ's judgment on rebellious Israel and the nations.

It's part of the same timeline as Revelation 18:

Revelation 18 describes the fall of Babylon, symbolic for Jerusalem. Revelation 19 follows immediately as the aftermath, celebrating God's vindication.

Revelation 19:11 is a vivid symbolic vision of Christ's coming in judgment on apostate Israel, fulfilled in the destruction of Jerusalem in 70 AD. It is not a future, physical, sky-splitting return, but a covenantal judgment using symbolic apocalyptic language, just as the Old Testament prophets used when God judged nations (Isaiah 13, Ezekiel 32, etc.).