The rainbow covenant was eternal (Gen 9:12), so God continues to determine the rising and falling of the nations during the New Testament age..
He made all the nations; and he marked out their appointed times in history and the boundaries of their lands (Acts 17:26).
When nations go sour and begin to magnify evil, God causes them to collapse and die. Sometimes they just collapse in on themselves, but often they destroyed by the armies of other nations.