USSR and US
The USSR had just passed the US only a couple of years earlier.
America has the most nuclear weapons
RussiaUS
As the category for this question is "Nuclear weapons", I'm presuming it refers to nuclear arms.As of 2012, the three countries with the largest stockpiles of nuclear weapons are, in order from most to least:RussiaUnited StatesFrance
Most did their own independent development.
Most countries have zero nuclear weapons.
No one. Most countries do not export nuclear weapons. Instead, the countries obtain (1) nuclear technology and (2) radioactive material and build it themselves.
building of nuclear weapons
As of 2012, the countries with the most nuclear weapons, counting actively deployed and stockpiled ones, are, in order of most to least:RussiaUnited StatesFrancePeople's Republic of China (mainland)United KingdomPakistanIsraelIndiaNorth Korea
Japan doesn't want any nuclear weapons.But Japan can easily produce nuclear weapons if it wanted to. It has the technology to build nuclear weapons within 1 year-- probably a few months if it wanted.However, most Japanese people are against nuclear weapons. Therefore, they do not have nuclear weapons.
From 1945 until the late 1970s or early 1980s the United States always had the most nuclear weapons. After then the USSR/Russia has had the most nuclear weapons.
Hard to say. Many countries make nuclear weapons behind other countries backs. Some countries are not announcing when they complete testing or development for nuclear warheads, so the world may never know.
Russia