Im not sure what hes talking about here, but I think we should drown the Iranians in a sea of fire and blood, the sooner the better. :^)
He's talking about the savagery that once characterized Western warfare, but which has been wall-papered over by our view that we should be more enlightened. However, strip away the comforts of our lives and threaten our existence, and the old savage will wake up and act. Combine that savagery and modern technology, and you can conjure up a pretty scary picture of what we can do. Hansen cited "The barbarity of the nightmares at Antietam, Verdun, Dresden and Hiroshima" as evidence of this. I'd add to it Aachen, Germany, where in early 1945 the US army got very tired of dealing with German snipers in a "kinder and gentler" fashion, and proceeded to use 155mm howitzers as anti-sniper weapons. We leveled 3/4 of the city, civilian casualties be damned. I'll also add our behavior after the German massacre at Malmedy during the Battle of the Bulge - the US Army didn't take a single German prisoner for weeks afterwards, the rules of war be damned.
In short, if someone on the other side breaks the rules often enough or badly enough, then we also do the same - with far better and more numerous weapons. Hansen is warning the Iranians not to do what Yamamoto knew that Japan did after the Pearl Harbour attack..."wake a sleeping giant and fill him with a terrible resolve."
What Iran may have forgotten is that the U.S. hasn't really gotten angry at anybody since then.