Here is the section that ends with the idea of weaponized asteroids:
On Earth, speciation can occur when spatially expanding populations become geographically isolated from one another (the unique diversity of species on the Galapagos Islands is the paradigm example of this phenomenon). Because of the vast distances between planets it is likely that similar fragmentation would eventually occur in outer space. Along with the interplanetary spread of cyborgs and artificial intelligences, the result, centuries after a viable Mars colony is established, will probably be a plethora of intelligent species, all of which will have evolved to fit their distinctive ecological constraints—an archipelago of politically distinct worlds. The idea is common in sci-fi, from Verner Vinge’s 1993 A Fire Upon the Deep to Mark Fergus and Hawk Otsby’s The Expanse, both Hugo Award winners.
Multi-world pluralism can look attractive if we assume that everyone will get along, regardless of the profound morphological, technological, and ideological differences that are bound to grow up around and between these groups. Here, however, the space expansionist invocation of natural selection bites back. Radiating and diversifying species notoriously compete with one another for available space and its resources or, in the case of intelligent species, just for glory and prestige. This is all familiar enough from the history of life on Earth, as is the mostly sorry result of the human interaction with other species as well as earlier human groups. We exterminated the Neanderthals (after breeding with them for a while) and are, according to the United Nations, currently in the process of eviscerating the non-human biosphere.
Doesn’t it seem likely that our deep-space descendants will inherit these destructive tendencies and turn them on each other? A space archipelago will be composed of mutually suspicious and competitive groups, millions of them eventually. But the bonds of sameness that can foster respectful recognition or mutual forbearance will surely diminish with increased interplanetary spatial dispersion and the ordinary workings of evolution. Not that we should expect a space-based Hobbesian war of all against all. There will doubtless be a good deal of room for interspecies and interworld diplomacy in this scenario. However, in the absence of a pacific trans-planetary government—and given our inability to create a single world government here, the chances of that seem slim—opportunities for plunder and general mayhem will likely abound. The temptation to cast the interplanetary Other as subhuman will be pronounced. Remember that the intelligent aliens in Starship Troopers are “bugs,” and in Battlestar Galactica they are “toasters.” Even in our fiction, it seems, we have a difficult time imagining what peaceful co-existence among wildly disparate beings might look like.
Because of this, all minimally viable colonies will have compelling reasons of state to stockpile awesome weapons of mass destruction: not only hydrogen bombs, but more importantly the ability to convert asteroids into planetoid bombs. Somehow this possibility—potentially genocidal or xenocidal wars of worlds—seems not to matter much to space expansionists, even though it’s standard fare in the well of sci-fi from which many of them have drunk so deeply.
Here is the section that ends with the idea of weaponized asteroids: