Adam Clarke on Haggai 2:6

Yet once, it is a little while, and I will shake the heavens - When the law was given on Mount Sinai, there was an earthquake that shook the whole mountain, Exo 19:18. "The political or religious revolutions which were to be effected in the world, or both, are here," says Abp. Newcome, "referred to; compare Exo 19:21, Exo 19:22; Mat 24:29; Heb 12:26-28. The political ones began in the overthrow of the Persian monarchy by Alexander, within two centuries after this prediction; and if the Messiah's kingdom be meant, which is my opinion, this was erected in somewhat more than five centuries after the second year of Darius; a short period of time when compared with that which elapsed from the creation to the giving of the law, or from the giving of the law to the coming of the Messiah's kingdom. It must be understood that the word אחת achath, once, has a clear sense, if understood of the evangelical age; for many political revolutions succeeded, as the conquest of Darius Codomanus, and the various fortunes of Alexander's successors; but only one great and final religious revolution." - Newcome.