It is a difficult book to accept for many people. It was difficult for me to accept.
But Joseph McCarthy had plenty of experience spotting people whose actions indicated that they might have conflicted allegiances, and Marshall drew his attention.
As he examined the life of George Catlett Marshall, he found reasons to suspect Marshall, for whatever reason, was serving the interests of entities other than the US Government.
McCarthy does not specify who or what those entities may have been, and he does not specify why he thinks Marshall may have been making decisions that served the interests of people who were not serving the interests of the United States of America.
He doesn't say Marshall is a Communist or Communist sympathizer, someone who might be influenced by ideology or a misguided desire to help the downtrodden. He doesn't say he thinks Marshall was being blackmailed. He doesn't address those things at all.
But what he does in this book is look at the indisputable facts as seen in a contemporary prism, while events are fresh, and examines Marshall's actions in it. He pointedly asks questions about those actions that many people don't want to even consider, much less answer.
Even to themselves.
He starts the book by baldly stating that people advised him against giving the speech in the Senate that the book is based on. This is on the first page of the book:
"...Those questions recalled the advice given me by some of my friends before I gave the history of George Marshall “Don’t do it, McCarthy,” they said. “Marshall has been built into such a great hero in the eyes of the people that you will destroy yourself politically if you lay hands on the laurels of this great man.” My answer to those well-meaning; friends was that the reason the world is in such a tragic state today is that too many politicians have been doing only that which they consider politically wise—only that which is safe for their own political fortunes..."
I think that in today's world, if one is a Patriot, that last sentence has to not only resonate, but serve as a familiar reminder of why we are in the position we are in.
As for the book, I will give you one scenario in the book that illustrates why McCarthy wrote the book, and why he had suspicion and contempt for Marshall:
"...Another item of interest in regard to Marshall is found in the Reader’s Digest of January 1944.
The late Frederick C. Painton was describing an interview had with General Marshall by 60 Anglo-American correspondents in Algiers:
“A door opened, a hush fell, General Marshall walked in. He looked around the room, his eyes calm, his face impassive. “To save time,” he said, “I’m going to ask each of you what questions you have in mind.” His eyes turned to the first correspondent. “What’s your question?“
A penetrating query was put; General Marshall nodded and went on to the next man—and so around the room, until 60 correspondents had asked challenging questions ranging from major strategy to technical details of the war on a dozen fronts. “General Marshall looked off into space for perhaps 30 seconds. Then he began.
For nearly 40 minutes he spoke. His talk was a smooth, connected, brilliantly clear narrative that encompassed the war. And this narrative, smooth enough to be a chapter in a book, included a complete answer to every question we had asked.
“But what astounded us most was this: as he reached the point in his narrative which dwelt upon a specific question, he looked directly at the man who had asked the question! “Afterward I heard many comments from the correspondents. Some said they had just encountered the greatest military mind in history. Others exclaimed over the encyclopedic detail Marshall could remember. All agreed on one thing: “That’s the most brilliant interview I have ever attended in my life.”
The above interview becomes extremely interesting when compared to Marshall’s inability to recall what he was doing on the morning of Pearl Harbor.
Originally, Marshall testified that he was out horseback riding and for that reason could not be contacted.
Later, he testified his memory had been refreshed and that he actually had not been horseback riding but was at home with his wife
The third version of where the Army Chief of Staff was on that fateful morning is contained in Arthur Upton Pope’s book Litvinoff, in which the diary account of Litvinoff’s trip from Russia to the United States shows that Marshall was meeting Litvinoff at the airport on Pearl Harbor morning. While the question of whether Marshall was riding horseback, or with his wife, or with Litvinoff seems unimportant today, it does form a very interesting comparison of Marshall’s memory on these two occasions..."
The reason I present this anecdote is that McCarthy's instincts on this were usually spot on when he was running his committee, and he had an ability to spot inconsistency. In this, he considered the seminal event of American history, the attack on Pearl Harbor, and knew that everyone from small children to elderly adults knew where they were on the morning of December 7, 1941. This is much the same for the Kennedy Assassination and the Challenger Explosion. It is burned into most of us.
And Marshall seemed not to know, changing his story under oath several times, and nobody called him on it.
Marshall was not a stupid man, as the anecdote about his press conference clearly shows. And given all McCarthy wrote in this book about the loss of China to the Communists, Marshall was not a man who made careless mistakes.
The last page of the book states:
"...If Marshall were merely stupid, the laws of probability would have dictated that at least some of his decisions would have served this country’s interest. Even if Marshall had been innocent of guilty intention, how could he have been trusted to guide the defense of this country further?
We have declined so precipitously in relation to the Soviet Union in the last six years, how much swifter may be our fall into disaster with Marshall’s policies continuing to guide us? Where will all this stop? This is not a rhetorical question; ours is not a rhetorical danger.
Where next will Marshall’s policies, continued by Acheson, carry us? What is the objective of the conspiracy? I think it is clear from what has occurred and is now occurring: to diminish the United States in world affairs, to weaken us militarily, to confuse our spirit with talk of surrender in the Far East and to impair our will to resist evil. To what end? To the end that we shall be contained and frustrated and finally fall victim to Soviet intrigue from within and Russian military might from without. Is that far-fetched? There have been many examples in history of rich and powerful states which have been corrupted from within, enfeebled and deceived until they were unable to resist aggression..."
I think the questions/mysteries you raised are mostly solved by reading the works on Anthony Sutton.
https://www.amazon.com/Wall-Street-FDR-Antony-Sutton/dp/1905570716
https://www.amazon.com/Wall-Street-Bolshevik-Revolution-Capitalists/dp/190557035X
https://www.amazon.com/Wall-Street-Hitler-Antony-Sutton/dp/0945001533
Follow the money.
A forensic audit of Marshall would answer all the questions.