Russia’s invasion of Ukraine is taking place in the shadow of the two largest nuclear arsenals in the world. These arsenals, built during the Cold War, persist as a very real threat, with the potential for a nuclear strike theoretically little more than the duration of an ICBM’s flight time away. This reality, of the ever-present risk from nuclear weapons, reemerged in the public consciousness on Sunday, February 27, when Russian President Vladimir Putin ordered the country’s nuclear forces placed on high alert.
The order, which Reuters reports was carried out by Russia’s defense ministry on Monday the 28, includes adding more personnel to the military departments responsible for launching nuclear strikes. Staffing increases are one way to signal a readiness to launch nuclear weapons as a first strike, or to prepare to launch those same weapons in response to another country’s first strike.
It is hard to know what, if any, other changes were made as part of Russia’s move. That’s in part because of the secrecy surrounding nuclear weapons complexes, which is a term for everything from the production and maintenance of nuclear weapons, to the placement, preparation, and command decisions regarding those weapons.
In response to Russia’s move to high alert, on February 28, President Joe Biden told a reporter that Americans should not worry about a nuclear war breaking out.
Nuclear weapons and command structures can be harder to understand than the movement of a weapon like tanks or artillery, but even when they are not used, their existence and readiness can shape the war in big ways. Here’s what to know.
What are nuclear weapons?
First demonstrated in New Mexico at the Trinity test site on July 16, 1945, nuclear weapons use explosives to compact dense nuclear fuel, usually uranium-235 or plutonium-239 isotopes, setting off a fission reaction. By using explosives to force these isotopes together, the warhead splits the nucleus of the atom which sets off the fission chain reaction, splitting more and more atoms and in the process releasing a tremendous amount of energy. Fission bombs, also called atomic bombs, were used in the Trinity test, as were the two bombs the United States dropped on Hiroshima and Nagasaki, Japan, in August 1945. The low estimate for the combined deaths in Hiroshima and Nagasaki is 110,000, and the high estimate is 210,000.
Modern nuclear weapons are different. They use both a primary stage, like a plutonium pit, to create a fission reaction, and a secondary stage that creates a fusion reaction. When a warhead has both of these components, it is also known as a thermonuclear or a hydrogen weapon, and this type of reaction can produce an explosion orders of magnitudes more powerful than the energy released in an atomic bomb.
The yield of “Little Boy,” the atomic bomb the US dropped on Hiroshima, was estimated at 15 kilotons of TNT, or the same as 15,000 tons of TNT. The B83 nuclear bomb, currently the most powerful thermonuclear weapon in the inventory of the United States, has a yield of 1.2 megatons of TNT, making it 80 times more powerful than Little Boy.
In addition to bombs and cruise missiles carried by planes, nuclear warheads can be launched by intercontinental ballistic missiles or by submarine-launched ballistic missiles. Some of these missiles can have multiple warheads per missile.
How many nuclear weapons are there?
The Federation of American Scientists estimates that there were a total of 12,700 nuclear warheads at the start of 2022. Those weapons are held by nine countries: the United States, Russia, the United Kingdom, France, China, Israel, India, Pakistan, and North Korea. South Africa once had nuclear weapons, but dismantled them in 1989 in anticipation of a change of government.
Russia inherited the nuclear arsenal of the former Soviet Union, which before it was dissolved in 1991 also stored nuclear weapons in Belarus, Ukraine, and Kazakhstan. Those warheads were all returned to Russia in the 1990s. Russia had the ability to maintain and authorize the use of those warheads, in part because Russians staffed nuclear divisions. Also, the actual controls preventing unauthorized use of nuclear weapons were held in Moscow.
Russia has the largest arsenal, which FAS estimates at 5,977 warheads, with the United States having the second-largest arsenal, at 5,428. China has the third largest at an estimated 350 warheads. The US’s fellow NATO defensive alliance members France and the United Kingdom each have 290 and 225 warheads, respectively.
Many of those warheads are held in reserve, for maintenance or potential future dismantlement. Russia and the US both have a smaller portion of their overall nuclear arsenal deployed at places like air force bases, at roughly 1,600 and 1,650 warheads, respectively. That is the baseline level of nuclear readiness between the two countries.
What does nuclear readiness mean?
Each country with nuclear weapons has its own command and control process that leaders use to decide if, when, and where to launch nuclear weapons. Informing this decision can be everything from early warning sensors—like satellites that can detect the flash of light from launches—to radars which can pick up incoming missiles. Because missiles carrying nuclear weapons move fast, many decisions about how to respond need to be made quickly.
Sometimes, decisions are made in the detection stage of the process. In 1983, Soviet officer Stanislav Petrov saw an early warning of a launch detected by a satellite, but reasoned that the number of launches detected was too few to actually indicate a surprise attack, and chose not to escalate the warning. It was later revealed that what the satellite computer interpreted as a launch was instead the reflection of the sun on a cloud.
The decision to launch weapons in retaliation to a detected strike is called “launch on warning,” while a decision to launch nuclear weapons after missiles hit is called “launch on impact.” Pavel Podvig, an arms control researcher, has argued that the Soviet Union had a formal policy of “launch on impact,” which structured how the country built its nuclear command and control. A launch detected by early warning would then give the Soviet leadership time to increase the readiness of its nuclear forces, and issue orders for possible retaliation, if the warnings were accurate.
It has been over 30 years since the dissolution of the Soviet Union, but Russia inherited the nuclear enterprise of the USSR, and it is possible the country has not updated its posture since. One possible meaning of a shift to high readiness is the physical connecting of circuits that allow a launch command to go through.
What does NATO have to do with all this?
Nuclear weapons are a devastating technology. Nukemap, a popular online tool to simulate blast radiuses and other effects from potential nuclear explosions, has been overwhelmed by traffic this week. Because of the short time between a nuclear launch and impact, political leaders often try to set expectations about what will and will not constitute a threat worthy of nuclear retaliation.
Even as the United States and other countries in NATO supply Ukraine with weapons and other aid for its fight against Russia’s invading armies, the Biden administration has been clear that the US will not directly fight Russian forces unless one of the NATO countries is attacked.
On February 11, before the invasion began and while Russia was still amassing forces to invade, Biden warned Americans in Ukraine to leave the country. Asked if there was a scenario where he would send US troops to rescue Americans in Ukraine, Biden said “There’s not. That’s a world war when Americans and Russia start shooting at one another.”