Splitting atoms - or nuclear fission - produces energy (in the form of heat) called nuclear energy. The key to controlling the splitting of atoms, thereby managing the power of nuclear energy, lies in knowing how, and when, to stop the chain reaction.
A controlled chain reaction produces energy; an uncontrolled chain reaction produces a nuclear explosion. Enrico Fermi, while working at the University of Chicago, discovered how to control a nuclear chain reaction.
In his nuclear reactor - the world's first - a life-changing event took place on the 2nd of December, 1942. Fermi and his team made history when they helped to produce the first self-sustaining, controlled nuclear reaction. Thereafter, nuclear energy (and nuclear weapons) were just over the horizon.
Today, power plants in various countries use nuclear reactors (instead of burning coal) to create electricity. The same concept applies to nuclear-powered submarines where the energy produced from nuclear reactions (not fossil fuel) powers the ship. Always at issue, among other things, is the safe disposal of nuclear waste.
The major difference between steam that is produced by a coal-fired boiler, for example, and steam that is generated by nuclear power is what happens inside the nuclear reactor.
In 1939, when Einstein sent his letter to President Roosevelt, scientists were still learning these things about nuclear energy. No one had yet built nuclear power plants, atomic bombs or nuclear-powered submarines.
At the time, people were using hydro-electric power to run their homes and businesses. Hoover Dam - a massive project to boost such power generation for America's southwestern region - was only three years old when Einstein wrote to FDR.
How did the President react to Einstein's concerns? Initially preoccupied with Hitler's invasion of Poland, which started World War II, FDR was proceeding cautiously. By fall, he asked for advice from the newly formed "Advisory Committee on Uranium" which first met on October 21, 1939.
The advice he received ultimately led to the "Manhattan Project" and the world's first atomic bombs.