Diamond's main theme is that societies have made choices about how they interact with the environment. Some choices are sustainable and some are not. Societies that survive make sustainable ecological choices. Diamond's theme is cautionary: he is arguing that our own culture's survival is not a given and that if we do not heed the environment, our society will sooner or later collapse.
Diamond, for example, points to the Norse settlers in Greenland. Their society lasted for hundreds of years, but ultimately was unsustainable because they refused to adapt to the ecology of Greenland. They imported their Scandinavian lifestyle and relied on cattle and other livestock. Eventually, they exhausted their natural resources and began to starve. However, rather than learn from the natives who had lived sustainably on Greenland for thousands of years through fishing in kayaks, the Norse, believing themselves innately superior, refused to adopt native ways. Eventually their society disappeared.
Diamond's book cries out against an arrogant approach to the environment. Humans can't simply dominate the environment forever in unsustainable ways and expect this to work because it's how they've "always" lived or because this is how they want it to be. Stupidity and willful blindness have sunk cultures before, and it can happen again. We need to recognize we are part of a bigger world of nature and understand that if we don't align ourselves with it, our culture will likely perish.