Truman relieved General MacArthur of his command over coalition forces in Korea in large part because he planned and aggressively advocated the use of nuclear weapons against the Chinese.
If you visit the Princeton stacks, you can unearth many Woodrow Wilson School theses advocating the aggressive use of nuclear weapons during Vietnam. Some of the people who authored these papers went on to craft US trade policy from the 1980s through the present. A truly remarkable change of heart.
This is still debated, and MacArthur later denied it in his testimony to the Senate Inquiry. His removal has a clearer and simpler reason: Truman felt MacArther's communication with the Chinese violated his December directive and preempted the President, while the Joint Chiefs' concern was that "if MacArthur were not relieved, a large segment of our people would charge that civil authorities no longer controlled the military." [1]
The simplest explanation for the removal is that MacArthur tacitly disobeyed the Commander-in-Chief (enraging President Truman).
[1] https://en.wikipedia.org/wiki/Relief_of_Douglas_MacArthur#Re...
You have to be careful with sentences like that. It removes all of the urgency and action from a discussion.
https://en.wikipedia.org/wiki/Dr._Strangelove
General MacArthur was dismissed during the Korean War because the White House weren't sure if he was going to use nukes or not (like the movie.) The policy was that the US was ready (bombers with nukes were positioned in Japan), but President Truman wanted to give final advance approval to commanders, yet MacArthur was issuing his own policy.
https://en.wikipedia.org/wiki/Douglas_MacArthur
(It turns out that MacArthur was right about basically everything he ever said, and continued to advise Presidents.)
It turns out the Pentagon Papers were supposed to be just a sideline. He also copied documents on US nuclear strategy, which was horrific. He released the Vietnam documents first because he figured if he started with the nuclear docs, nobody would care about the Vietnam stuff.
He gave the nuclear papers to his brother to hide. His brother hid them first in his compost pile, then got worried and moved them to a garbage bag on the edge of the town dump. The next day, men in suits were seen poking metal rods in his compost pile.
Unfortunately, a freak storm washed away that section of the dump. Ellsberg and his brother spent a year opening garbage bags in the area where it all ended up, to no avail. Ellsberg said his wife considered the storm a gift from God, because if the nuclear papers had hit the light of day, he surely would have spent the rest of his life in prison.