The Pentagon, unsentimental and politically conservative, might not seem likely to be worried about climate change—still linked, for many people, with polar bears and coral reefs.

Yet of all the major institutions in American society, none take climate change as seriously as the U.S. military. Both as participants in climate-triggered conflicts abroad, and as first responders to hurricanes and other disasters on American soil, the armed services are already confronting the impacts of global warming. The military now regards climate change as one of the top threats to American national security—and is busy developing strategies to cope with it.

Drawing on previously obscure reports and government documents, renowned security expert Michael Klare shows that the U.S. military sees the climate threat as imperiling the country on several fronts at once. Droughts and food shortages are stoking conflicts in ethnically divided nations, with “climate refugees” producing worldwide havoc. Pandemics and other humanitarian disasters will increasingly require extensive military involvement. The melting Arctic is creating new seaways to defend. And rising seas threaten American cities and military bases themselves.

While others still debate the causes of global warming, the Pentagon is intensely focused on its effects. Its response makes it clear that where it counts, the immense impact of climate change is not in doubt.