Sometimes Brits don't seem to really understand the American Civil War. Bernard Cornwell's Starbuck series set within that war, for instance, it's so bad -- and, yes, wrong. He's dropped writing it too. It clearly wasn't winning readership -- this doesn't happen to Cornwell, but this time it did.
McPherson was more kind than the WaPo reviewer who revealed that Keegan stated, iirc, in the Introduction to his book, that he really doesn't understand why the United States had this civil war. That's pretty profound non-comprehension, which for the life of me, I cannot figure out why, since it's very simple to understand. Slavery was the root cause. The determination of the confederacy to expand the trade and institution as the economic platform for the U.S. throughout the hemisphere, not to mention the already existing North American states. This was not to be tolerated by the industrial capitalists of the Northern states. They had their own vision of manifest destiny, and there was no place for this kind of slavery in it.
On the other hand, what McPherson more kindly takes Keegan to task about is his profound ignorance of our geography, since his central argument is that this is war as geostrategics. McPherson has this to say about Keegan's analysis:
The analytical value of Keegan’s geostrategic framework is marred by numerous errors that will leave readers confused and misinformed. I note this with regret, for I have learned a great deal from Keegan’s writings. But he is not at top form in this book. Rivers are one of the most important geostrategic features he discusses. “The Ohio and its big tributaries, the Cumberland and the Tennessee,” he writes, “form a line of moats protecting the central Upper South, while the Mississippi, with which they connect, denies the Union any hope of penetration.” The reality was exactly the contrary. These navigable rivers were highways for Union naval and army task forces that pierced the Confederate heartland, capturing Nashville, New Orleans, Memphis and other important cities along with large parts of Tennessee, Louisiana, Mississippi, Alabama and Arkansas. Keegan acknowledges this reality later in the book when he notes that these rivers “offered points of penetration to the Union into Confederate territory.” Precisely.