Eden, Daniel refers to Israel as "the beautiful land," (= "land of the Decoration," NW, see Dan. 8:9; 11:16, 41, 45), not as "the whole earth."
Of all the nations of that time, Rome would utilize iron more than any before. Archaeologists call the period "the middle iron age." Rome made extensive use of iron whereas some of the older nations such as the Greek City states were still advancing from Bronze weapons. This is why they got beaten. The superiority of Roman weapons and tactics would subdue "all the inhabited earth."
This is what the Keil-Delitzsch Commentary had to say:
Dan. 7:23. Fourth kingdom. Daniel receives the following explanation regarding the fourth beast. It signifies a fourth kingdom, which would be different from all the preceding, and would eat up and destroy the whole earth. “The whole earth is the ο?κουμ?νη ,” the expression, without any hyperbole, for the whole circle of historical nations.
These four kingdoms are the Babylonian, the Medo-Persian, the Macedo-Grecian, and the Roman. This was an opinion held by Josephus (Ant. 10.10.4 §209). He viewed the third layer coming from the west, identifying it with Greece, so that the fourth should be Rome. This was also the conclusion reached by Martin Luther: “In this interpretation and opinion,” he observes, “all the world are agreed, and history and fact abundantly establish it.” This would be the case, until the end of the nineteenth century.
The relation of the world-kingdoms to the kingdom and people of God, represented by the gradation of the metals, correspond only to the Babylonian, Medo-Persian, Macedo-Grecian, and Roman world-kingdoms, but not to the Babylonian, Median, and Persian. First the Roman kingdom spread its power and dominion over the whole ο?κουμ?νη , over all the historical nations of antiquity in Europe, Africa, and Asia. “There is” (says Herodian, ii. 11. 7) “no part of the earth and no region of the heavens whither the Romans have not extended their dominion.”
Still more the prophecy of Daniel reminds us of the comparison of the Roman world-kingdom with the ear lier world-kingdoms, the Assyri o-Baylonian, the Persian, and the Macedo- Grecian, in Dionys. Halicar., when in the proaem. 9 he says: “There are the most famous kingdoms down to our time, and this their duration and power. But the kingdom of the Romans ruled through all the regions of the earth which are accessible, inhabited by men; it ruled also over the whole sea, and it alone and first made the eas t and the west its boundaries.”