In his classic study of Psychological Warfare, Paul Linebarger offers historical examples of successful and unsuccessful psychological warfare. Milton's support for the Cromwellian regime calls into the latter category:

“Milton fell into the common booby-trap of refuting his opponents item by item, thus leaving them the strong affirmative position, instead of providing a positive and teachable statement of his own faith. He was Latin Secretary to the Council, in that Commonwealth of England which was—to its contemporaries in Europe—such a novel, dreadful, and seditious form of government. The English had killed their king, by somewhat offhanded legal procedures, and had gone under the Cromwellian dictatorship. It was possible for their opponents to attack them from two sides at once. Believers in monarchy could call the English murderous king-killers (a charge as serious in those times as the charge of anarchism or free love in this); believers in order and liberty could call the British slaves of a tyrant. A Frenchman called Claude de Saumaise (in Latin form, Salmasius) wrote a highly critical book about the English, and Milton seems to have lost his temper and his judgment.”

Linebarger argues that Milton “committed almost every mistake in the whole schedule of psychological warfare. He moved from his own ground of argument over to the enemy's. He wrote at excessive length. He indulged in some of the nastiest name-calling to be found in literature, and went into considerable detail to describe Salmasius in unattractive terms. He slung mud whenever he could. The books are read today, under compulsion, by Ph.D. candidates, but no one else is known to find them attractive. It is not possible to find that these books had any lasting influence in their own time.”

His failure as a propagandist was a blessing to English literature: “Milton turned to disappointment and poetry; the world is the gainer.”