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Assessment

Historian John H. Lavalle argues that:

Marlborough's place as one of the finest soldiers Britain ever produced is well deserved. He possessed the personal courage, imagination, common sense, self-control, and quick wits that mark the best battlefield commanders. He had an unerring ability to sense an enemy's weaknesses and the ability to use stratagems to throw his enemy off balance.... It was in the realm of strategy, however, where Marlborough shone. As commander-in-chief of a coalition army, he could tolerate politicians, allies, and fools gladly. He also saw the potential of the recently introduced combination of flintlock and socket bayonet to restore the offensive...to warfare in an age when extensive fortifications, magazines, and the defensive dominated military thinking. Marlborough's attention to logistics allowed him to break free of the bonds of the magazine system and ensured that his soldiers were fed, clothed, and paid, earning him the loyalty of his troops and the nickname "Corporal John".

Marlborough was equally adept at both battle and siege. Robert Parker writes:

in the ten campaigns he made against; during all which time it cannot be said that he ever slipped an opportunity of fighting when there was any probability of coming at his enemy: and upon all occasions he concerted matters with so much judgment and forecast, that he never fought a battle which he did not gain, nor laid siege to a town which he did not take.

To military historians David Chandler and Richard Holmes, Marlborough is the greatest British commander in history, an assessment that is shared by others, including the Duke of Wellington who could "conceive nothing greater than Marlborough at the head of an English army". However the 19th century Whig historian Thomas Macaulay denigrated Marlborough throughout the pages of his History of England. In the words of historian John Wilson Croker he pursued the Duke with "more than the ferocity, and much less than the sagacity, of a bloodhound", though according to G. M. Trevelyan Macaulay "instinctively desired to make Marlborough's genius stand out bright against the background of his villainy". It was largely in response to Macaulay that a descendant, Winston Churchill, wrote his laudatory biography, Marlborough: His Life and Times (4 vol. 1933–1938).

Marlborough was ruthlessly ambitious, and relentless in the pursuit of wealth, power, and social advancement, earning him a reputation for avarice and miserliness. These traits may have been exaggerated for party faction but, notes Trevelyan, nearly all other statesmen of the day were engaged in founding families and amassing estates at the public expense; Marlborough only differed in that he gave the public much more value for their money. In his quest for fame and personal interests he could be unscrupulous, as his desertion of James II testifies. To Macaulay this is regarded as a piece of selfish treachery against his patron; an analysis shared by G. K. Chesterton, a devout Catholic:

Churchill, as if to add something ideal to his imitation of Iscariot, went to James with wanton professions of love and loyalty... and then calmly handed over the army to the invader. To the finish of this work of art but few could aspire, but to their degree, all the politicians of the Revolution were upon this ethical pattern.

To Trevelyan, Marlborough's behaviour during the 1688 revolution was a sign of his "devotion to the liberties of England and the Protestant religion". However, his continuing correspondence with Saint-Germain was not noble. Although Marlborough did not wish for a Jacobite restoration his double-dealing ensured that William III and George I would never be fully disposed to trust him.

Marlborough's weakness during Anne's reign lay in the English political scene. His determination to preserve the independence of the Queen's administration from the control of the party faction initially enjoyed full support, but once royal favour turned elsewhere, the Duke, like his key ally Godolphin, found himself isolated; first becoming little more than a servant of the Whigs, then a victim of the Tories. Nonetheless he has been consistently ranked as one of the greatest military commanders of the modern era.