The growing destructive capacities of humanity make this the prime problem of the twenty-first century. How we cope with this problem will have a profound bearing on the world of our grandchildren. The twentieth century was the bloodiest ever. World War II caused at least 50 million deaths. Six million died in the Holocaust. At Hiroshima, one bomb caused 100,000 deaths. Now thousands of such bombs (smaller and more conveniently transportable tactical bombs) are housed in Russia. Many are poorly guarded, susceptible to theft and bribery. Others may be made elsewhere. Danger increases with the number of such weapons existing in the world.Why? There is a greater chance of error, theft, and bribery—and ultimately their use in war or terrorism. Therefore, we should diminish the numbers as much as we can and secure responsible stewardship for those that remain. Moreover, there are still thousands of nuclear weapons that are far more powerful than the smaller tactical weapons. Biological and chemical weapons are easier to make than nuclear warheads and therefore have special appeal to terrorists. Small arms and light weapons now cover the world wall-to-wall. They include highly lethal machine guns, mortars, automatic rifles, and rocket launchers. Altogether, the destructive capacity of humanity is almost beyond imagination. Moreover, there is an exciting effect of today’s vast weapons on political demagogues, religious fanatics, and ethnic haters—and plenty of them exist in the world. Incitement to hatred and violence can occur with radio, TV, the Internet, and many other media. Thus, we can more powerfully incite violence, utilize more lethal weapons, and do much more damage than ever imaginable before. No group is so small or so far away as to prevent it from doing immense damage anywhere. The time has come to move beyond complacency, fatalism, denial, and avoidance. We must urgently seek to understand and strengthen an array of institutions and organizations that have the capacity to use tools and strategies to prevent deadly conflict. The first author (D.A.H.) considered many such possibilities in his recent book, No More Killing Fields: Preventing Deadly Conflict. Overall, this gives humanity a greater range of possibilities than ever before for building a system for preventing war and genocide. It will not be easy.