Albert Barnes on Revelation 9:12

One woe is past. The woe referred to in Rev 9:1-11. In Rev 8:13, three woes are mentioned which were to occur successively, and which were to embrace the whole of the period comprised in the seven seals and the seven trumpets. Under the last of the seals, we have considered four successive periods, referring to events connected with the downfall of the Western empire; and then we have found one important event, worthy of a place in noticing the things which would permanently affect the destiny of the world--the rise, the character, and the conquests of the Saracens. This was referred to by the first woe-trumpet. We enter now on the consideration of the second. This occupies the remainder of the chapter, and in illustrating it the same method will be pursued as heretofore: first, to explain the literal meaning of the words, phrases, and symbols; and then to inquire what events in history, if any, succeeding the former, occurred, which would correspond with the language used.

And, behold, there come two woes more hereafter. Two momentous and important events that will be attended with sorrow to mankind. It cannot be intended that there would be no other evils that would visit mankind; but the eye, in glancing along the future, rested on these as having a special preeminence in affecting the destiny of the church and the world.