Hiroshima gets all the attention, but Nagasaki teaches the more important lesson. The need to destroy Hiroshima will be forever debated, but the counterarguments were unpersuasive to President Harry Truman and Secretary of War Henry Stimson. A world war had taken the lives of tens of millions. Noncombatants were not spared. When a war-ending weapon was finally available – too late to make unnecessary the Normandy landing, but just in time to substitute for the invasion of Japan’s home islands – Truman and Stimson chose to end the carnage as soon as possible.

The arguments in favor of the first explosive use of an atomic bomb do not apply to the second. Japan’s War Cabinet was absorbing the twin shocks of Hiroshima and Russia’s declaration of war against Japan. At a minimum, Truman and Stimson should have waited more than three days before obliterating Nagasaki and killing its inhabitants. The argument used to justify the fate of Nagasaki was that Japan’s dead-enders needed to know that more atomic bombs would rain death and destruction unless they surrendered. This justification is not persuasive because everyone understood that the immense machinery of U.S. war production would be working overtime to make more atomic bombs, and that it was just a matter of time when they would rain more destruction over Japan.

The need to surrender would sink in after Hiroshima and the Russian announcement. Would this take three days, five or ten? Whatever: After Hiroshima, it was worth the wait. That Nagasaki was sacrificed without waiting is a testament to the inexorable danger inherent in war plans involving nuclear weapons. Truman and Stimson chose not to intervene with their agreed plan to keep up the bombing until Japan surrendered. The United States possessed two A-bombs and detonated two A-bombs. If three were available, and if the Emperor was unable or unwilling to assert himself over dead-enders, then a third city would have been targeted.

The fate of Nagasaki demands that leaders delve into nuclear war-fighting plans. They rarely do. Before assuming office, newly elected U.S. Presidents receive briefings on the nuclear codes and the “football” that will become constant company, but these briefings are more about process than substance. Presidents usually don’t dwell on targets, since there are so many of them as to be incomprehensible. The natural human reaction to even the briefest introduction to Armageddon is to shudder inwardly and to hope fervently that targeting plans remain in locked safes.

Because nuclear weapons have not been used on battlefields since Nagasaki, it is safe to presume that this instinct has been widely shared – and not just by leaders, but also by those who found themselves well down the chain of command at terrible junctures in our nuclear history – those who looked bleakly into the abyss without the means or the time to check with higher authority. We now know the names of some of these heroes. One is Vasili Alexandrovich Arkhipov, who chose not to fire a nuclear-armed torpedo while his submarine was being depth-charged to the surface during the Cuban missile crisis. Any human being who does not recoil at the point of decision to fire a nuclear weapon is, by definition, the most dangerous person on the planet. And yet nuclear war-fighting plans are predicated on these decisions.

The second most important line of defense against mushroom clouds is an intuitive understanding that controlling escalation once the nuclear threshold has been crossed is very likely to be a complete fiction. Leaders in the United States, Russia, and Pakistan who continue to assert the right of first use do so only by clinging to this extraordinarily thin reed.

Once the first mushroom cloud appears in a contest between nuclear-armed combatants, pressures to retaliate in kind will be immense. And once these Gates of Hell have been opened, mere mortals are likely to be powerless to close them. Mushroom clouds do not open lines of communication that have broken down, resulting in warfare. Under what pretense, then, do U.S. and Russian leaders insist on having four-digit-sized nuclear arsenals? What will Chinese, Pakistani, and Indian leaders do with three-digit-sized arsenals if a mushroom cloud appears by accident, miscalculation, or fateful decision?

The historical example of Nagasaki speaks volumes about how hard it is leaders to grind the machinery of warfare to a halt once the first mushroom cloud appears. Nagasaki therefore demands our attention as much as Hiroshima. The fundamental lesson of Nagasaki is that a second nuclear detonation follows the first.

On the 71st anniversary of Nagasaki, Barack Obama and Vladimir Putin can spend no better time than to take a very hard look at the nuclear war-fighting plans their armed forces have prepared. And then pick up the phone to agree on parallel reductions in their massive nuclear arsenals.

Michael Krepon is Co-Founder of the Stimson Center. This piece originally ran in USA Today on August 7, 2016.