That may make some heads spin but read his report here
Here is an excerpt:
" ...2010 Quadrennial Defense Review (QDR), where the Pentagon noted that: “While climate change alone does not cause conflict, it may act as an accelerant of instability or conflict, placing a burden to respond on civilian institutions and militaries around the world.” Then former secretary of defense Leon Panetta noted in 2012 before the Environmental Defense Fund:
In the 21st century, the reality is that there are environmental threats which are threats to our national security. For example, the area of climate change has a dramatic impact on national security: rising sea levels, to severe droughts, to the melting of the polar caps, to more frequent and devastating natural disasters all raise demand for humanitarian assistance and disaster relief."
Of course, if runaway greenhouse gases overheat our planet and melt the ice caps, foreign armies will be the least of our problems. A dead planet is hard to defend and is bad for business since there won't be any business at that point...