Everyone lost their minds when a Marine general relieved an Army general

It wasn't the first time - in WWI, Marine MajGen John A. Lejeune relieved an Army general and took over command of the 2nd U.S. Division (Regular), which probably didn't make General Pershing too happy, since he didn't like Marines much and didn't think they belonged in France. So LtGen "Howlin' Mad" Smith's actions in firing an Army division commander for what he considered to be "insufficient aggression" on the battlefield, regardless if he had reconnoitered the terrain himself, were entirely justified, despite the fact that the Army and the Marine Corps operational doctrines were then (and still are) carried out in different ways. With LtGen Smith being in overall command of the Saipan action (and not much of a fan of Army procedures), he had the right to appoint and fire whomever he pleased in order to get the Saipan operation advanced the way he wanted. As far as I know, that still applies in joint operations today, no matter who is in overall command. Inter-service politics has no place on a battlefield.

Many Generals in American history have been fired for being too cautious, inept, unable to act with aggressiveness, and not being able to command a division.

I gave an NCO development class on this. If any and all branch's are working together and they fall in a combined arms operations with set chains of command an Army regiment could fall under the command of the Navy or Marines hence all UCMJ action would fall to them not the Army. Oh lord you would have thought I farted in in church with the poo poo faces. LOL