As so often, the best (most convincing) explanation I have seen of Zephaniah is in the Catholic Encyclopaedia, which says:
The threatening of the “day of the Lord”, a Dies irae dies illa … as a punishment for the awful degeneracy in religious life (i, 4-7a); it will extend to all classes of the people (i, 7b-13), and will be attended with all the horrors of a frightful catastrophe (i, 14-18); therefore, do penance and seek the Lord (ii, 1-3).
Not only over Jerusalem, but over the whole world (urbi et orbi), over the peoples in all the four regions of the heavens, will the hand of the Lord be stretched…
With a special threat (3:1-8)
The Prophet then turns again to Jerusalem: “Woe to the provoking, and redeemed city. . . She hath not hearkened to the voice, neither hath she received discipline”; the severest reckoning will be required of the aristocrats and the administrators of the law … and of the Prophets and priests, as the directors of public worship.
A …prophetic glance at the Kingdom of God of the future, in which all the world, united in one faith and one worship, will turn to one God, and the goods of the Messianic Kingdom, whose capital is the daughter of Sion, will be enjoyed.
Similar themes are woven throughout the prophets of the Old Testament. Unless ‘the people’ repent, the anger of God will be vented on the whole population. But general repentance in time will lead to the Kingdom of God.
With the benefit of hindsight, God did not in historical fact wreak havoc on an international scale as an expression of his anger, either at the time of Zephaniah or since. At least, I have not heard it seriously advanced as a proposition that Hiroshima was the result of God’s anger at mankind in general.
In my lifetime, I have been told I would be submerged in the population explosion (I wasn’t), not have enough to eat as agriculture would not be able to keep up (it did); be blown to smithereens by an atom bomb (I wasn’t); be killed in a terrorist incident (I haven’t been); drown as the Thames flooded its banks (I wasn’t); be part of a collapsing world at the millennium thanks to the Y2k bug (I wasn’t); and that my children or grandchildren would die as a result of climate change. Of course, there is still time for me to be killed by any or all of the above. But over the years, I have tended to regard reports of imminent Armageddon as over-stated.
I also do not believe destruction on an enormous scale to be part of God’s nature, in fact we have a covenant with Him which says so. We each have a personal relationship with God, and I accept a share of our collective guilt for polluting our planet and risking its survival. So the message of Zephaniah, in a digital age, is, I suggest, not a clash of cymbals and kettle drums, but a still small voice which asks each one of us whether we need to repent for our collective sins against our world.