Sixty six million years ago a mountain sized chunk of rock, traveling at more than 10 times the muzzle velocity of an assault rifle, slammed into the shallow sea covering what is now the Yucatan Peninsula of Mexico. Shortly thereafter the 5th of the Earth’s great mass extinctions occurred. The energy released by the impact was comparable to having a 1-megaton nuclear explosion spaced every 5 km over the Earth’s surface. Massive tidal waves and earthquakes swept the Gulf of Mexico, and rock ejected from the crater carpeted the land as far away as Wyoming. However, a greater danger lay in the vaporized asteroid and coral from the bottom of the sea, which flew back into space in a massive fireball. After the material in the fireball recondensed into drizzle-sized droplets of rock it reentered the atmosphere over the entire Earth as a massive swarm of shooting stars. The shooting stars heated the upper atmosphere until it glowed like the heating bar in an electric broiler oven. The radiation from this hot gas and rock ignited all the land biomass on Earth, creating firestorms on a global scale, and broiled the dinosaurs alive. Then it became pitch-black, to dark even for cats to see, and much too dark for photosynthesis. Temperatures on the land fell below freezing. In the few years before the Earth recovered many species of plants and animals that were not broiled alive were driven to extinction by the cold weather. The food chain in the ocean collapsed because phytoplankton could no longer metabolize without sunlight, and the fish, ammonites and other larger creatures that depended on plankton starved to death. Many of these same phenomena may occur if there is a nuclear war. Nuclear explosions in cities would cause urban firestorms, killing large numbers of people. For example, 20 million Indians and Pakistanis might die directly from nuclear bomb blasts in a war involving 2/3 of their current arsenals. North Korea could cause as many casualties as the U.S. experienced in World War II, by exploding only 3 small atomic weapons over U.S. cities. The smoke from these firestorms would rise into the upper atmosphere, block sunlight, and cool the planet. The resulting cold temperatures would reduce or eliminate food production. Even a war between smaller powers, such as India and Pakistan, could cool the Earth to lower temperatures than any in thousands of years, damaging agriculture and triggering mass starvation, perhaps killing a billion people. A war between Russia and the US would likely create sub-ice age temperatures and kill the majority of the human population by starvation. Destruction of agriculture, starvation and large losses of life would sweep the planet even in places not attacked. Unfortunately, at present, we are not capable of stopping an asteroid from hitting the planet. It remains to be seen if we can prevent a nuclear conflict.