13 And the legs and feet of the image were of iron. This is the kingdom of the children of Shem, who are the children of Esau, which is strong as iron. And he said:— As iron breaks and subdues everything, so also the fourth kingdom shall break and bruise everything. And he explained with reference to the feet and toes, that part of them was of iron and part of them of potter's clay. For he said:— Thus they shall be mingled with the seed of man, and they shall not cleave one to another, as iron cannot be mixed with clay. This referred to the fourth kingdom.
Because in the kingdom of the children of Esau there was not a king, the son of a king, established to govern the kingdom; but when the children of Esau were gathered together into a powerful city, then they made a senate. And from thence they used to set up as chief of the city a wise man to govern the kingdom, lest when the Governor of their kingdom should weigh them, they might be found wanting, and the kingdom might be taken away from them as the kingdom of the children of arrogant Nimrod was taken away and given to the children of foolish Madai.
And this king who was set up, the seed of that former king was destroying him; and they did not cleave one to another. But as to the seed of man which is compared with the clay, the meaning is this; that when the king was chosen for the kingdom, he mingled himself with the root of the kingdom of iron.
Source: Demonstrations (New Advent)