The former Soviet Union began work on its nuclear weapons program in the 1940s after hearing reports of the US Manhattan Project.
After the Soviet-US arms race during the Cold War, nuclear weapons previously stored in former Soviet states were returned to Russia where many were dismantled. But Russia still maintained a vast stockpile of weapons.
Today, Russia appears to be investing in nuclear weapons modernization — much like the US — and growing its arsenal. Last year, Obama criticized such efforts as impediments to global nuclear disarmament.
"Because of the vision that he's been pursuing of emphasizing military might," Obama said, referring to Russian President Vladimir Putin, "we have not seen the type of progress that I would have hoped for with Russia."
In October, Putin said he wants to help reduce the world's nuclear arsenal and "will be striving to achieve that," but added that Russia will continue to develop its program so long as other countries continue doing so.
While Russia has the most nuclear weapons of any country in the world, that doesn't necessarily mean they are the most powerful.
"Russia built nuclear weapons that are incremental improvements," or weapons that would need updating every decade or so, Jeffrey Lewis, the founding publisher of Arms Control Wonk, told Business Insider.
On the other hand, Lewis said, "US nukes are like Ferraris: beautiful, intricate, and designed for high performance. Experts have said the plutonium pits will last for 100s of years." Indeed, the US's stocks of Minuteman III ICBMS, despite their age, are "exquisite machinery, incredible things."
"Russia's nuclear weapons are newer, true, but they reflect the design philosophy that says 'No reason to make it super fancy because we'll just rebuild it in 10 years,'" Lewis added.
United States: 6,800
The US ushered in the nuclear era under President Franklin Roosevelt in 1942 when the military launched the Manhattan Project, which led to the world's first nuclear bomb detonation test.
During World War II, the US forever changed the way the world would look at nuclear technology after dropping bombs on Hiroshima and Nagasaki in Japan, instantly killing tens of thousands of civilians.
The US is a member of the NPT, but it has refused to sign onto a no-first-use policy.
Earlier this year, former Vice President Joe Biden doubled-down on major investments to boost America's nuclear capabilities.
"So long as other countries possess nuclear weapons that could be used against us, we too must maintain a safe, secure, and effective nuclear arsenal to deter attacks against ourselves and our allies," Biden said. "That is why ... we increased funding to maintain our arsenal and modernize our nuclear infrastructure."
Quartz reported that the US will spend approximately $400 billion over a 10-year period to maintain and modernize its arsenal. Another purpose of this investment is to keep pace with Russia's growing arsenal as well.
"I want modernization and total rehabilitation," the president said. After calling for an increase in the US stockpile on the campaign trail, he said in October 2017 that would be "totally unnecessary."
France began developing nuclear weapons during the Cold War, when President Charles de Gaulle believed it needed defense capabilities independent of the US and NATO. De Gaulle feared that neither would come to France's defense in the event of an attack by the Soviet Union or some other enemy.
While France possesses the third-largest nuclear weapons stockpile in the world, it claims it has no chemical or biological warfare weapons. It is a member of the NPT.
In 2008, French President Nicholas Sarkozy reaffirmed that the country's nuclear weapons were not "targeted at anybody." Rather, they were part of a "life-insurance policy." Sarkozy also announced a nuclear weapons reduction, cutting its stockpile to "half the maximum number of warheads [France] had during the Cold War.
China's first nuclear weapons test took place in 1964. Like India, Beijing maintains a no-use-first nuclear policy, but some in the international community are skeptical of its intentions.
Beijing keeps its nuclear weapons count secret, so it's impossible to determine exactly how many the country has. While the East Asian superpower is a member of the NPT, its increasingly ambitious military ventures have been a cause of concern for some countries.
Next year, for example, China plans to unveil its next-generation intercontinental ballistic missile, which will be able to strike anywhere in the world and carry up to 10 nuclear warheads. In 2016, similar long-range nuclear missiles capable of striking Guam, a US territory, were revealed, sending shockwaves through the American defense establishment.
United Kingdom: 215
Like all other nuclearized countries, the UK argues that it needs nuclear weapons largely for defense purposes.
Its nuclear weapons deterrent is called Trident and consists of four Vanguard-class submarines that can carry up to 16 Trident II D5 ballistic missiles, each armed with up to eight nuclear warheads, The Telegraph reported.
From 2010 to 2015, the UK cut the number of its operational warheads by 40, from 160 to 120. It continues to work on nuclear reduction while maintaining its advocacy for minimum nuclear force — just the right amount of force to inflict devastation and achieve combat goals.
Contrary to India's no-first-use policy, Pakistan has not ruled out first-attack use of nuclear weapons.
The 1971 Indo-Pakistani War and the threat of India's burgeoning nuclear weapons capabilities prompted Pakistan to start a nuclear program of its own.
In 2014, Pakistan began developing tactical nuclear weapons, which are smaller warheads built for use on battlefields rather than cities or infrastructure. These weapons are small enough to launch from warships or submarines, which makes them easier to use on short notice than traditional nuclear weapons.
Pakistan is also reportedly nearing completion of its nuclear triad, which would give the country the ability to launch nuclear missiles from the land, air, and sea.
To put it mildly, India has a hostile relationship with its neighbor, Pakistan. That tension is compounded by the fact that both countries possess nuclear weapons. For nearly two decades, however, the two nations have avoided any escalating nuclear conflict.
In 2003, India, which is not a party to the NPT, declared a no-use-first policy, meaning that it vowed to never use nuclear weapons in combat unless first attacked by another country with nuclear weapons. China maintains a similar policy.
India first began developing nuclear weapons in an attempt to counter Chinese aggression in the 1960s. It has since tested multiple nuclear devices, which caused the US to impose, then later lift, various sanctions.
Israel's government will neither officially confirm nor deny it has nuclear weapons. But it's an open secret that the Middle Eastern country has been building nuclear weapons for decades.
In 1986, Mordechai Vanunu, a former nuclear technician and whistleblower, revealed the existence of Israel's program.
Western allies, like the US and the UK, have supported Israel's policy of keeping its program "secret."
The Guardian reported that in 2009, when a reporter asked Obama if he knew of any country in the Middle East with nuclear weapons, "he dodged the trapdoor by saying only that he did not wish to 'speculate.'"
For years, the US tried to negotiate with North Korea to curb its nuclear weapons program. The Agreed Framework, signed in 1994 under President Bill Clinton, ultimately failed. North Korea was cheating.
In 2003, Pyongyang officially withdrew from the NPT. Three years later, the country tested its first nuclear test. North Korea has since continued building weapons, despite efforts by Presidents George W. Bush, Barack Obama, and now Donald Trump to slow its progress.