This is the beginning of a new part of the story in which Matthew describes the beginning of Jesus's ministry in Galilee. These verses explain how Jesus came to be in Galilee. (See: [[rc://en/ta/man/jit/writing-background]])
This can be stated in active form and the information omitted from the euphemism can be stated. Alternate translation: "the king had handed John over to the prison" or "the king had arrested John" (See: [[rc://en/ta/man/jit/figs-activepassive]] and [[rc://en/ta/man/jit/figs-euphemism]])