And the fourth kingdom shall be strong as iron: forasmuch as iron breaketh in pieces and subdueth all things: and as iron that breaketh all these, shall it break in {u} pieces and bruise.

(u) That is, the Roman empire will subdue all these others, which after Alexander were divided into the Macedonians, Grecians, Syrians, and Egyptians.

Wesley's Notes

2:40 Fourth kingdom - This is the kingdom of the Romans, and was to last not only to Christ's first coming, but under antichrist, to his second coming. This did break in pieces all other kingdoms, being too strong for them, and brought all into subjection to it, 'till the stone fell upon it.

Jamieson-Fausset-Brown Bible Commentary

40. iron-This vision sets forth the character of the Roman power, rather than its territorial extent [Tregelles].

breaketh in pieces-So, in righteous retribution, itself will at last be broken in pieces (Da 2:44) by the kingdom of God (Re 13:10).

Matthew Henry's Concise Commentary

2:31-45 This image represented the kingdoms of the earth, that should successively rule the nations, and influence the affairs of the Jewish church. 1. The head of gold signified the Chaldean empire, then in being. 2. The breast and arms of silver signified the empire of the Medes and Persians. 3. The belly and thighs of brass signified the Grecian empire, founded by Alexander. 4. The legs and feet of iron signified the Roman empire. The Roman empire branched into ten kingdoms, as the toes of these feet. Some were weak as clay, others strong as iron. Endeavours have often been used to unite them, for strengthening the empire, but in vain. The stone cut out without hands, represented the kingdom of our Lord Jesus Christ, which should be set up in the kingdoms of the world, upon the ruins of Satan's kingdom in them. This was the Stone which the builders refused, because it was not cut out by their hands, but it is become the head stone of the corner. Of the increase of Christ's government and peace there shall be no end. The Lord shall reign, not only to the end of time, but when time and days shall be no more. As far as events have gone, the fulfilling this prophetic vision has been most exact and undeniable; future ages shall witness this Stone destroying the image, and filling the whole earth.