Personally, I believe that Daniel was written much later than it claims and much of the "dream" was already fulfilled. Like everything else in the Bible, theories abound.
The first four kingdoms have been identified as the Babylonian, Persian, Greek and Roman Empires. Daniel already said that Babylon, specifically Nebuchadnezzar, was the head of gold. Babylon fell to the kingdom of the Medes and the Persians. Greece became the successor to the Medo-Persian Empire. The “iron” empire can only be Rome.
Opinions differ on the fifth empire. Some have tried to identify various periods in Europe’s history as the clay-and-iron feet; others claim the feet represent the divided remnants of Rome before supposedly being “conquered” by Christianity. Still others believe that the clay/iron empire is yet to come: the kingdom of the Antichrist will be a “revived Roman Empire.”