At the most basic level, military operations directly consume vast quantities of fossil fuels and other resources. The use of powerful bombs, incendiaries, defoliants, etc. inflict substantial damage to the ecological network along with vast human suffering. The work to reconstruct cities and villages then consumes further resources.
Apart from these direct costs, military power enforces a system of global production and trade driven by remote concentrated centers of power rather than sensitivity to local needs and costs. Regimes are propped up that serve the needs of these remote powers, at great costs to the land and people of the region.
In addition to this, a system of global consumerism is cultivated in order to promote industrial productivity and technological progress, which are the cornerstones of modern military power. The culture of guns and violence seems to work to maintain a population ready to go to war.
Such self-reinforcing destructive patterns do not require any evil genius or conspiracy behind them. It's just the nature of things, like the way thunderstorms arise spontaneously. Understanding these patterns, though, may well lead us to find ways to disentangle ourselves from them!