Long before international Conventions agreed that humiliating and degrading treatment of prisoners should be forbidden, it was not only common to degrade prisoners but actually an integral part of the punishment.
And so it was with the Crucifixion of Jesus, a victim with such a high profile, well, it's not surprising the Roman soldiers took full advantage of the opportunity to show how tough they were.
(Has civilisation progressed much?)
After scourging Jesus, the soldiers placed a scarlet robe over His shoulders and a crown of thorns on His head. To complete the regal image, they put a rod in His right hand as a sceptre, then with a mocking bow cried out, "Hail, King of the Jews!" (Matt. 27:29.)
It is likely that the rod was a reed baton, similar to sticks used by security forces today for controlling unruly crowds. In the next verse (Matt. 27:30), we read how the soldiers then took the rod and beat Jesus about the head, no doubt pushing the thorns further into his scalp.
The soldiers were, of course, absolutely correct is addressing Jesus as King. Unbeknownst to them, they were also fulfilling a prophecy of Jesus death (Matt. 20:17-19).