Every large organization, especially nations and civilizations, is held together by a mythology, or narrative. What they do and how they do it is driven by the foundational narrative that instructs all of the participants.
Organizations fall when some inherent/foundational tenets in their narrative eventually come up against an uncooperative physical reality. Often, this is because the physical world has changed, and the myths now contradict the new reality. When that happens, you’d think the primary task of an organization’s elites is to manage the modification of the narrative myth to accommodate the new reality and continue as a renewed/redefined entity thereby.
However, what they typically do is try to save the narrative at all costs. That narrative, after-all, is what places them in a privileged position, and so they have a vested interest in keeping it alive. Change is necessarily going to place their position at risk. As long as fundamental concepts need only modification, this can be handled. If fundamental concepts have to be jettisoned, the enterprise and their place in it is necessarily subject to far greater risk of going out of control. I suspect this is largely what happened to the Soviet Union as it tried to modify its fundamental principles. It went out of control, and was hi-jacked in a weak moment. So, the result is inertia until a dam breaks somewhere and the floodwaters carry the whole thing away.
The USM’s mythology/narrative problem is a fractal of the West’s much larger mythology/narrative problem. The whole Western enterprise is founded on a 500+ yr old mythology that is running headlong up against new realities. Its ideas of Progress, of Exceptionalism, of being the pinnacle of Development of Justice and Governance, are running into the realities of resource depletion, economic and martial decline, and a lazy, decadent & corrupt political elite, as against competitors who are on the opposite trajectory. To the the extent that the Pentagon’s mythology was written in the context of a surging West, it borrows much of Western mythology. The Pentagon hasn’t re-written its mythology, and probably can’t until the West re-writes the larger version.
In classical physics, inertia is proportional to mass. As the largest organization on the planet (or at least employer), it shouldn’t surprise us that the US DoD exhibits the most organizational inertia.