"A Civil War is not only the conflict of opposing principles but the shock of material forces."
Sir Charles Firth.
The causes of the Civil war have been fought over by historians for centuries. So much so that not for nothing did Lawrence Stone describe it as "a battleground which has been heavily fought over…beset with mines, booby-traps and ambushes manned by ferocious scholars prepared to fight every inch of the way."
Kenyon's book, which is one of the better military histories, does not confine itself to a straightforward, matter of fact description of the civil war. It is an excellent piece of history, with a deep understanding of the politics and economics of the conflict which he combined with his military understanding.
Kenyon's work was praised by Christopher Hill who said Kenyon's narrative was "orthodox Tawneyite: towns and cities 'solidly for Parliament', so much so that in 1643 Charles I insisted that he 'dared not trust his person inside any closed town'; the clothing areas were 'aggressively parliamentarian', Birmingham 'a solidly parliamentarian industrial town'. Kenyon grasps the fact which 'revisionist' historians ignore, that before 1642 social revolution was always potentially present". As we shall see later, Kenyon did not return Hill's compliment.
The book, while being well researched, has the pace of an excellent novel. It is methodically researched as to be expected of a historian of Kenyon's experience. As one writer states, he had "scholarly attention to detail and ability to extract every nuance from his sources. He distrusted fads and was sceptical of theories not fully backed by the historical fact".
It is not for nothing he was described as "one of Britain's leading scholars of 17th-century English history". Kenyon sought to position himself between the two camps and tried to place his book within the context of the civil war being a product of a general European Crisis of the 17th Century. This view tends to cut across the mainly nationalist English view of the civil war. (See Eric Hobsbawm-The General Crisis of the European Economy Past and Present No 5 May 1954 pp 33-53).
It is hard as regards historiography to fit Kenyon into a discernable category. Robert Ashton makes an interesting point on why some historians while not being Marxist did use Marxist ideas saying "The idea of religious, political and constitutional issues as an ideological superstructure based on foundations of material and class interests has been influential far beyond the ranks of Marxist historians. It has indeed been adopted, in part at least and with a radically different emphasis, by some of their more formidable and determined opponents".
As Christopher Thompson forcibly wrote Kenyon was not a Marxist or fellow traveller but believed there was a "Great Rebellion" not an English revolution. However, Kenyon’s use of this quote from Sir Charles Firth that "a Civil War is not only the conflict of opposing principles but the shock of material forces." tends to confirm Ashton's perceptive analysis.
Kenyon's use of the quote by James Harrington, "the dissolution of this government caused the war, not the war the dissolution of this government" is further confirmation that Kenyon was a very thoughtful historian more than many have given him credit.
Harrington is a significant figure if you are looking for a materialist understanding of the English revolution. The writer and historian Gaby Mahlberg makes this perceptive point "'Good laws', Harrington believed, could give the country stability, and these laws had to be infallible, so that bad man would not be able to corrupt the state. Harrington never saw his dream come true. The Restoration of the Stuarts in 1660 meant a return of many of the old problems. However, his ideas of mixed government and a balance of power remained influential in the writings of the Neo-Harringtonians of the later 17th and early 18th century. They influenced both the American and French Revolutions, while his materialist theory of political change would also strike a chord with Marxists and modern economic and political thinkers". Kenyon was not a Marxist, but Harrington certainly struck a chord with him.
High Tide of Revisionism
The fact that Kenyon had no discernable historiography meant that many could claim him for their brand of history. Leading one obituary writer to say "When he appeared at Christ's in 1954, he cast himself in the role of mocking outsider, offering caustic criticisms from the fringes of college power in the confident and correct expectation that they would mostly be ignored. They were. College meetings would be punctuated by Kenyon's heavy sighs and even heavier disapproving sniffs and brief, dismissive comments, but the college men of affairs went about their efficient business untroubled by these background mutterings".
Kenyon's book appeared when writing about the English revolution was an extremely hazardous occupation. Kenyon was too independent of mind to call himself a revisionist, but this did not stop the dean of revisionism John Morrill claiming him for their side.
John Morrill for one believed that Kenyon was close to the revisionists and made it abundantly clear in his obituary for the British Academy that Kenyon had the "fundamental disapproval of model-builders and systematisers. He had no time for social determinism as a tool of the historian for explaining the past or of social engineering as a tool of the politician in effecting the future". 
To say that Kenyon was a mass of contradictions would be an understatement but sometimes he went too far. His review of Christopher Hill's The World Turned Upside Down was not only wrong, rude and disrespectful as this quote shows:" I think we are entitled to ask where all this discussion of obscure left fanatics is getting us. That some of them were mad, we have always impatiently known, but Dr Hill positively glories in it. This was not a proletarian movement at all. It was an unexpected opportunity for failed shopkeepers, lazy artisans and eccentric historians to find their voice".
Kenyon rejected being dragged into the Revisionist or post revisionist camp as his article Revisionism and Post Revisionism in Early Stuart History shows. His attack on Conrad Russell's reluctance to see the revolution as a clash of social classes scolding "to Russell, then, the crisis was one of central organisation: how to control the three kingdoms in a war situation; how to solve the church problem when each nation had a majority espousing a different faith from the other two, each with a substantial minority inclined toward the faith of another kingdom; and how to secure a financial settlement adequate for early modern government. The effect was to create a bewildering number of new axes of division. Russell has never allowed that this was a struggle between social classes, constitutionalism and absolutism, between Court and Country, or between "government" and "opposition".
To conclude it is perhaps fitting and generous on my part to allow John Morrill to have the last word on Kenyon when he wrote" John Kenyon had the best historical intelligence of his generation. He understood men and women in the past and wrote about them with a rare precision, clarity and conviction. He was a productive scholar, and all his works except one wore their learning with a deep deceptive lightness. He fitted into no school, reacted against fashion, came to look old fashioned in his interests. He was a magnificent historian who could not quite build on the brilliance of his early promise, but who greatly underestimated the magnitude of his own achievement and the continuing appeal of his writing".
 Continuity and Anachronism: Parliamentary and Constitutional Development by P.B.M. Blaas
 The Civil Wars of England--John Kenyon - Weidenfeld & Nicolson, 1988 - xvi + 272pp- https://www.historytoday.com/christopher-hill/civil-wars-england
 The Civil War and the Class Struggle-Robert Ashton-1970
 A History of England, Volume 1: Prehistory to 1714- By Clayton Roberts, F David Roberts, Douglas Bisson
Proceedings of the British Academy (Volume 101 (1999), pages 441-461) https://www.thebritishacademy.ac.uk/sites/default/files/101p441.pdf
 Review: Revisionism and Post-Revisionism in Early Stuart History- The Journal of Modern History, Vol. 64, No. 4 (Dec., 1992), pp. 686-699
 Proceedings of the British Academy (Volume 101 (1999), pages 441-461) https://www.thebritishacademy.ac.uk/sites/default/files/101p441.pdf