Many and varied. Lets take two perspectives. The first is that climate change is considered a threat multiplier. That is, it is an additional thing to deal with by countries already under / experiencing stress. It can increase instability through water shortages, food (in)security, extreme events, degrading critical infrastructure, placing pressure on health systems and so on. This can lead to instability, to increase tensions, and in extremis to conflict. So, this is clearly an important element for militaries that need to weigh-up what they need to be prepared for, how they train, what type and balance of capability they have (and so on). The other dimension for militaries are impacts on their own infrastructure, bases, estate and so on. As a collective, militaries forces are worlds largest land holder and capital holder; sprawling, vast, global, and employing missions. So, climate change impacts this also - militaries are already assessing and evaluating this.