The Cost of War Project has calculated that the US military has produced a total of 1.2 billion metric tons of greenhouse gasses since the beginning of the global war on terror in 2001. Of that, 400 million metric tons are directly due to war related fuel consumption. Full report is here.
Today China is the world’s largest greenhouse gas emitter, followed by the United States. In 2017 the Pentagon’s greenhouse gas emissions totaled over 59 million metric tons of carbon dioxide equivalent. If it were a country, it would have been the world’s 55th largest greenhouse gas emitter, with emissions larger than Portugal, Sweden or Denmark.