Revelation 20:1
Revelation 20:11Then I saw an angel coming down from heaven with the key to the Abyss, holding in his hand a great chain.
Watchtower commentary takes as the opening of a literal future thousand-year period. After Armageddon, an angel binds Satan in the abyss for exactly 1,000 years; during this period Christ and the 144,000 rule from heaven over a paradise earth populated by surviving Witnesses and the resurrected dead. The binding has not yet begun and is the next major future event after the great tribulation.
Reformed readings (amillennial, partial-preterist) hold that the binding has already begun. Jesus said in that he could not be casting out demons unless he had first bound the strong man — present tense at his first coming. The Greek "bind" verb in and shares the same root. The purpose of the binding in ("that he might not mislead the nations anymore") is exactly what changed at Pentecost: the gospel exploded out of Jerusalem to the Gentile nations who had been kept in darkness (, 17:30). The Watchtower's strictly future binding requires denying that Christ's first coming actually inaugurated his kingdom — a position no traditional Christian reading shares.