Flavius Josephus

"Now some of the Jews thought that Herod's army had been destroyed by GOD, and that it was a very just penalty to avenge John, surnamed the Baptist. For Herod had killed him, though he was a good man, who bade the Jews practise virtue, be just one to another and pious toward GOD, and come together in baptism. He taught that baptism was acceptable to GOD provided that they underwent it not to procure remission of certain sins, but for the purification of the body, if the soul had already been purified by righteousness. And when the others gathered round him (for they were greatly moved when they heard his words), Herod feared that his persuasive power over men, being so great, might lead to a rising, as they seemed ready to follow his counsel in everything. So he thought it much better to seize him and kill him before he caused any tumult, than to have to repent of falling into such trouble later on, after a revolt had taken place. Because of this suspicion of Herod, John was sent in chains to Machaerus, the fortress which we mentioned above, and there put to death. The Jews believed that it was to avenge him that the disaster fell upon the army, GOD wishing to bring evil upon Herod."

Flavius Josephus - Jewish historian