See Daniel ii. 31-40. "And the fourth kingdom shall be strong as iron; for as much as iron breaketh in pieces and subdueth all things." The remainder of the prophecy (the mixture of iron and clay) was accomplished, according to St. Jerom, in his own time.
Sicut enim in principio nihil Romano Imperio fortius et durius, ita in fine rerum nihil imbecillius: quum et in bellis civilibus et adversus diversas nationes, aliarum gentium barbararum auxilio indigemus (Opera, tom. v. p. 572).