Key point: It is no wonder that Japan feels very invested in U.S.-North Korean negotiations.

Tokyo has a population of 13.491 million, making it by far the largest city in Japan and home to more than 10 percent of the nation’s population. The greater Tokyo area, home to thirty-seven million, is the largest urban area on the planet. It is also squarely in the crosshairs of North Korea’s nuclear weapons. In the event of nuclear attack, Tokyo could experience enormous destruction the likes of which it hasn’t experienced since World War II.

Tokyo became the capital of modern Japan in the late nineteenth century, during the Meiji Restoration, part of a broad effort to modernize the country and raise the Japan’s government, science, technology and military to Western levels. Unfortunately, imperialism was one of those imported values, and the country ruled the Korean Peninsula from 1910 to 1945. The occupation and guerrilla war conducted by Korean partisans established a deep antagonism against Japan that persists to this day. The North Korean leadership claims legitimacy in part from the partisan activities conducted by founding members of the state. Japan’s later alliance with the United States and South Korea only exacerbated bad feelings against the country in Pyongyang.

Despite such history, in practical terms Japan has only been a secondary player to Pyongyang, with the real enemy being the United States. Kim Il-sung, founder of the North Korean state and grandfather to current leader Kim Jong-un, set a goal of building long-range missiles to strike Japan—not for striking Japan per se, but for striking American military bases there that would be key to a new Korean conflict. Kim wanted a way to strike bases that would host or coordinate the kind of American airpower that flattened Pyongyang during the war.

Most American bases in Japan are near urban areas, a consequence of Japan’s shortage of useful land. The U.S. Air Force has also closed other bases such as Tachikawa Air Base, which was located in the western part of Tokyo. One base in particular, however, that would very likely be struck by North Korean nuclear weapons is Yokota Air Base.

Yokota Air Base is home to fourteen thousand American servicemen and women. It is home to the headquarters for U.S. Forces Japan and an airlift wing, and is a vital operations, transportation and medical base for the U.S. military in East Asia. It is also the location of the Japan Air Self Defense Force’s Air Defense Command, and a base from which U.S. Air Force B-29 bombers staged attacks on North Korean targets during the Korean War.

In the event of war, some experts are convinced that North Korea would launch a broad nuclear attack designed to shock its enemies, prove a will to use its nuclear arsenal and warn enemies not to provoke further attacks. Yokota, a joint U.S.-Japanese base with a history of hosting attacks against the North Korean people, would almost certainly become a target.

What would happen if Yokota was struck with a North Korean nuclear weapon? Using NUKEMAP, an online tool developed by nuclear-weapons scholar Alex Wellerstein, we can gain some kind of idea. For the purpose of this exercise, we’ll assume North Korea is able to fit a twenty-kiloton nuclear warhead into a medium-range ballistic missile such as the Pukkuksong-2 missile and accurately deliver the weapon to the Yokota Air Base tarmac.

NUKEMAP provides ballpark guesses on a variety of nuclear-weapons effects, including thermal, overpressure and fallout effects, over various distances from the location of the detonation. In the case of Yokota, casualty estimates for a device 25 percent more powerful than the one detonated at Hiroshima are surprisingly mild: 12,800 killed outright and 45,460 injured. While this might seem unusual, the base is fairly large and very few people live or work on the flight line. A swathe of radioactive fallout would fall like a shadow to the northeast, across Saitama Prefecture and towards Ibaraki Prefecture, but generally avoiding metropolitan Tokyo.

What about other targets? A strike against the Japanese national government itself, most of which is located in Tokyo’s Kasumigaseki District, would kill 24,290 people outright and injure another 90,780. A cloud of radioactive fallout would again stream northeast, remain lethal as far as Asakusa Station, and Japanese civilians would need to be evacuated as far as Ibaraki and the coastline.

Perhaps the most dangerous strike in Japan would be against Shinjuku Ward, home to Japan’s Ministry of Defense. Here, a North Korean device would kill 56,400 outright and injure 128,310, and the radiation cloud would be the most devastating, spreading over the most densely populated areas of Tokyo.

The numbers behind NUKEMAP are not an exact science, and the site itself warns that estimates should be considered “evocative and not definitive.” It is unknown whether or not the model incorporates the “shielding effect” that causes high-rises to soak up blast effects, shielding buildings and people behind them—a particularly relevant factor for a city like Tokyo. Casualties from a nuclear attack could be higher or lower than the NUKEMAP estimate, but at least the site gives us an objective baseline.

Other factors unique to Tokyo could affect casualty rates. Tokyo’s world-famous transportation system incorporates hundreds of miles of underground tunnels. Functioning as ad hoc fallout shelters, these shelters could accommodate large numbers of people, something that would cause a dramatic drop in casualty rates if enough warning were given. On the other hand, Tokyo’s relatively flat topography will ensure a line of sight to the blast from miles away, possibly causing blindness and other injuries to those unfortunate enough to witness the attack.

Tokyo could be struck by multiple nuclear weapons, but North Korea has more targets than weapons at this point, and multiple strikes are unlikely. North Korea would also have Japan’s layered ballistic-missile defenses, consisting of SM-3 interceptors on Aegis destroyers backstopped by the Patriot PAC-3 missile system, to contend with. Still, as North Korea’s arsenal of deployable weapons gradually grows, so would the likelihood that the capital of Pyongyang’s historical enemy will be on the receiving end of multiple missiles. Finally, larger nuclear weapons would produce correspondingly higher casualty rates: a sixty-kiloton attack on Kasumigaseki would kill 56,710, almost two and a half times as much as a twenty-kiloton device, and injure nearly a quarter million people.

A nuclear attack on Tokyo would kill tens of thousands, cause serious injuries to even more and cause long-term health problems for millions. While ghoulish, the estimates are a reminder of the enormous power of nuclear weapons and the utter necessity that they never be used again.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami. This article first appeared last year.

Image: Reuters