Jamieson, Fausset and Brown on Revelation 12:15-16

15-16. flood--Greek, "river" (compare Ex 2:3; Mt 2:20; and especially Ex 14:1-31). The flood, or river, is the stream of Germanic tribes which, pouring on Rome, threatened to destroy Christianity. But the earth helped the woman, by swallowing up the flood. The earth, as contradistinguished from water, is the world consolidated and civilized. The German masses were brought under the influence of Roman civilization and Christianity [Auberlen]. Perhaps it includes also, generally, the help given by earthly powers (those least likely, yet led by God's overruling providence to give help) to the Church against persecutions and also heresies, by which she has been at various times assailed.