Albert Barnes Commentary 1 Kings 10:26

"And Solomon gathered together chariots and horsemen: and he had a thousand and four hundred chariots, and twelve thousand horsemen, that he bestowed in the chariot cities, and with the king at Jerusalem." — 1 Kings 10:26 (ASV)

See the note on 1 Kings 4:26. Until the time of Solomon, war chariots had not been used by the Jews, except to a very small extent (1 Chronicles 18:4). Therefore, it was necessary for him to put himself on an equal footing with neighboring powers in this regard.

Cities for chariots — These were likely fortresses on the borders of his territory, where he maintained the standing army necessary to support his dominion.