in Marx's time, the primary view of history was that its unraveling was guided by the decisive actions of "great men" who stepped up to the task when the times called for it. Dialectical Materialism says that its actually the material conditions of the world, specifically focusing on the conflict between the ruling class and the working class, and that whatever individuals happen to be the ones who lead are actually mostly irrelevant. In reality, both ideas are wrong though, since the system which decides what we'd consider "global human history" is incredibly complex, and from a bunch of research we've done since the 60s or so, its become clear that it doesn't make sense to talk in terms of 'cause' and 'effect' for these sorts of systems, because as the famous example goes, "changing something as insignificant as when a butterfly flaps its wings can ultimately end up deciding whether or not a hurricane occurs", and certainly nobody would suggest the butterfly has the ability to cause hurricanes. There's much more useful ways to look at these sorts of things, and methodologies I wish the left would adopt but I mostly feel like I'm yelling at a wall in vein