Charles Ellicott Commentary Daniel 2:40

"And the fourth kingdom shall be strong as iron, forasmuch as iron breaketh in pieces and subdueth all things; and as iron that crusheth all these, shall it break in pieces and crush." — Daniel 2:40 (ASV)

And the fourth. — It should be observed that the description of this kingdom is much fuller than those of the preceding empires. The same fact will be remarked in the later visions (Daniel 7:7–8; Daniel 7:19–20).

Breaketh all things. — Remembering that the comparison is between iron and the fourth empire, this portion of the vision implies that the Roman empire, which is here intended (see Exc. E), will crush out all traces that remain of preceding empires, just as iron is capable of breaking gold, silver, or copper. Of the second and third empires, each borrowed something from that which preceded it. The fourth empire introduces a new system, and a new civilisation.