A brief simplified look at Daniel 2. The interpretation of the dream was given to Daniel by God, Daniel 2:19-23. Daniel then gives Nebuchadnezzar the interpretation.
Daniel 2:37-40 (KJV): "37 Thou, O king, art a king of kings: for the God of heaven hath given thee a kingdom, power, and strength, and glory. 38 And wheresoever the children of men dwell, the beasts of the field and the fowls of the heaven hath he given into thine hand, and hath made thee ruler over them all. Thou art this head of gold. 39 And after thee shall arise another kingdom inferior to thee, and another third kingdom of brass, which shall bear rule over all the earth. 40 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 pieces and bruise."
Here is a prophetic time-line of history. Four successive kingdoms or world empires. Nebuchadnezzar is the head of gold, representing Babylon.
Daniel 2:39 (KJV): "And after thee shall arise another kingdom inferior to thee". Daniel does not state here what kingdom would replace Babylon, but the historical record states that the Medes took the kingdom:
Daniel 5:30-31 (KJV): "30 In that night was Belshazzar the king of the Chaldeans slain. 31 And Darius the Median took the kingdom, being about threescore and two years old."
Daniel 2:39 (KJV): "and another third kingdom of brass, which shall bear rule over all the earth." Again Daniel does not define the third kingdom. Daniel 8 reveals this in another vision, the Ram and the Goat, where the kingdom of Medo-Persia is replaced by Greece.
Daniel 8:20-21 (KJV): "20 The ram which thou sawest having two horns are the kings of Media and Persia. 21 And the rough goat is the king of Grecia: and the great horn that is between his eyes is the first king."
Daniel 2:40 (KJV): "40 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 pieces and bruise." Daniel's prophecy does not give a direct interpretation of this, but Luke gives us some idea.
Luke 3:1 (KJV): "Now in the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judaea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of Ituraea and of the region of Trachonitis, and Lysanias the tetrarch of Abilene".
The fourth kingdom is the Roman Empire.
Daniel 2:41-43 (KJV): "41 And whereas thou sawest the feet and toes, part of potters’ clay, and part of iron, the kingdom shall be divided; but there shall be in it of the strength of the iron, forasmuch as thou sawest the iron mixed with miry clay. 42 And as the toes of the feet were part of iron, and part of clay, so the kingdom shall be partly strong, and partly broken. 43 And whereas thou sawest iron mixed with miry clay, they shall mingle themselves with the seed of men: but they shall not cleave one to another, even as iron is not mixed with clay."
Much could be discussed on the detail here, but briefly the Roman Empire declined and what we have today is some strong and some weak or brittle elements.
Daniel 2:44-45 (KJV): "44 And in the days of these kings shall the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people, but it shall break in pieces and consume all these kingdoms, and it shall stand for ever. 45 Forasmuch as thou sawest that the stone was cut out of the mountain without hands, and that it brake in pieces the iron, the brass, the clay, the silver, and the gold; the great God hath made known to the king what shall come to pass hereafter: and the dream is certain, and the interpretation thereof sure."
The Kingdom of God will replace the present kingdoms of men when Christ returns. The stone initially hits the image on the feet, corresponding with the present era. When Jesus returns we can be either identified with the world, the kingdoms of men or with the stone, the Kingdom of God John 17:6,14-17, Matthew 21:42-44.
Kind regards
Trevor