Read Daniel 2:43 Using Other Translations
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.
As you saw the iron mixed with soft clay, so they will mix with one another in marriage, but they will not hold together, just as iron does not mix with clay.
This mixture of iron and clay also shows that these kingdoms will try to strengthen themselves by forming alliances with each other through intermarriage. But they will not hold together, just as iron and clay do not mix.