The US Department of Defense recognizes global climate change as a destabilizing influence, and therefore something to defend against. It isn’t their job to change climate change, but it is their job to defend the US from the indirect influence of climate change. They’re identified a significant list of influences.
Global climate change will have wide-ranging implications for U.S. national security interests over the foreseeable future because it will aggravate existing problems—such as poverty, social tensions, environmental degradation, ineffectual leadership, and weak political institutions—that threaten domestic stability in a number of countries. – US DoD
The primary region of contention is the potential land and sea disputes arising from the resources becoming accessible as arctic ice melts. Russia, Canada, Denmark, and the US are all big influences, and the details of the territories will hopefully be resolved diplomatically. If not, that’s one of the interests the Department of Defense gets to defend.