Gregory the Great — as excerpted in the Catena Aurea on Luke 21:9-11

“But that the end will not immediately follow these evils which come first, it is added, These things must first come to pass; but the end is not yet, &c. For the last tribulation is preceded by many tribulations, because many evils must come first, that they may await that evil which has no end. It follows, Then said he unto them, Nation shall rise against nation, &c. For it must needs be that we should suffer some things from heaven, some from earth, some from the elements, and some from men. Here then are signified the confusions of men. It follows, And great earthquakes shall be in divers places. This relates to the wrath from above.”