FROM THE first days of the atomic age, nuclear weapons have been designed to be aimed at two kinds of targets — military, and cities and industry. The only time the weapons were used in war, by the United States at Hiroshima and Nagasaki, they were dropped on cities. In the Cold War that followed between the United States and the Soviet Union, the majority of strategic nuclear weapons on intercontinental ballistic missiles were aimed at military targets, known as counterforce, while fewer were pointed at cities and industry. But the city-busting targets loomed large in the public mind and in the deterrence concept known as mutual assured destruction, in which the two superpowers faced each other in a cocked-pistols standoff.

The atomic bomb as a city-buster has always inspired terror. Fortunately, in the past two decades, these massive stockpiles have been radically reduced. So why would anyone want to go back to the era of nuclear fear? That is the question that hangs over the disclosure that Russia has been developing a nuclear-armed, underwater, unmanned drone. The new weapon was revealed when Russian President Vladimir Putin met with military chiefs in Sochi in November and television news footage captured a page being used in the briefing. The Kremlin later said the video showing “Ocean Multipurpose System ‘Status-6’ ” should not have been broadcast, and the video was deleted, but by that time it had gone viral — and global.

Russia appears to be creating a tactical nuclear weapon that could be slipped into a harbor, unleashing a tidal wave as well as the devastating effects of a nuclear explosion. It might be used to attack a military target, such as a submarine or naval base, but cities and industry could also be hit. According to the video, the mission of the proposed system is: “Damaging the important components of the adversary’s economy in a coastal area and inflicting unacceptable damage to a country’s territory by creating areas of wide radioactive contamination that would be unsuitable for military, economic, or other activity for long periods of time.” There are no arms control treaties in place to stop this; smaller tactical nuclear weapons have never been limited by treaty. And it is true that the United States, Russia and China are all modernizing nuclear and conventional forces.

The Russian drone now on the drawing board may reflect Mr. Putin’s oft-expressed desire to counter the U.S. antiballistic missile system with an asymmetric weapon. If so, this is a particularly dangerous choice. It could expand the threat of nuclear weapons into a whole new area. Unfortunately, there won’t be much debate about the drone in Moscow, where the news media and parliament are largely under Mr. Putin’s control and little scrutiny exists of his military adventures.