-"Oh king,
The image's legs of iron are the fourth kingdom.

Just as iron is strong and breaks things into pieces, and is stronger than other things,

so this kingdom will rule like iron and break in pieces all that try to fight against it."
Since we know the fulfillment of actual history, we know that this fourth kingdom was Rome.

Rome is symbolized here by two legs  of iron: Was Rome ever divided into two parts? Yes! In its decline it was divided  into an eastern and Western Empire.

The Western Empire had its capital at Rome, & the Eastern Empire called the Byzantine Empire, had  its capital at Constantinople.

Rome was the strongest of all these kingdoms. That's why it is symbolized here by iron.

Ad why was Rome not only the strongest, like iron, but also like a pair of legs? Because they were great on marching!

They were the first World Empire to build a lot of paved roads. They built big stone highways everywhere & the main purpose of their paved roads  was so their Roman legions could march rapidly to conquer any country that would rebel or cause them any trouble.