If the half hour of 8:1 is about 21 years, there was some time to prepare. Some insight into the preparation of God's people is outlined in chapters 7, 10, 11:1-12 & 14:1-12. The Destroyer (9:11) probably was also preparing. The sixth seal (6:12-17) was devastating, but sounds mild compared to the destruction during the first woe of Trumpets (8:6-9:12) to say nothing of the woes to follow!
See Matthew 24:31. There is a good chance that one purpose of the trumpets, in Revelation 8:2 to 9:21, is to call God's people to the place prepared for them! Some of God's people are to be kept from the hour of trial. God needs some to stand up to Satan; others may need to be heated up & refined first -- re-read the letters to the churches.
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Even if events in the past seem to fit the outline presented in the book of Revelation, that does not preclude a future series of events following the pattern outlined. Events tend to repeat -- follow somewhat similar patterns. Therefore, sometimes it is difficult to determine which events correspond to specific passages in the Bible.
First Trumpet: Hail & Fire Mixed With Blood
Second Trumpet: Blazing Mountain Thrown Into The Sea
Third Trumpet: A Great Star Fell From The Sky
Fourth Trumpet: Light Output Darkened By One-Third
Eagle Calling Out, "Woe! Woe! Woe!"
Fifth Trumpet: The Destroyer
Sixth Trumpet: 200,000,000 Troops Kill a Third of Mankind
The Open Book: Tasted Sweet But Upset Stomach
The Two Witnesses
Seventh Trumpet: You Have Begun To Reign