It was 1939, the year after the discovery of nuclear fission.
Szilárd, a Columbia University researcher, had found that fission could be used to generate electricity, but conversely used to generate a powerful nuclear chain reaction, most likely for a bomb.
The best material for this reaction was Uranium. The best source of uranium ore was in the Belgian-Congo. Nazi Germany was on the rise, and Szilárd was concerned they would get the bomb first.
First, he grabbed his friend and fellow physicist Eugene Wigner.
Wigner knew Albert Einstein, who knew the Belgian Royal Family.
They talked to Einstein, who agreed, Szilárd wrote the letter, and Einstein signed it.
Szilárd then leveraged his network in order to talk to economist Alexander Sachs, who could request a meeting with President Franklin Roosevelt.
Another letter, written, rewritten, then finally signed by Einstein, and delivered by Sachs, just as Germany invaded Poland.
Michael B. Stohff of The University of Texas describes the letter best:
The result was the “Einstein Letter,” which historians know as the product not of a single hand but of many hands. Regardless of how it was concocted, the letter remains among the most famous documents in the history of atomic weaponry. It is a model of compression, barely two typewritten, double-spaced pages in length. Its language is so simple even a president could understand it. Its tone is deferential, its assertions authoritative but tentative in the manner of scientists who have yet to prove their hypotheses. Its effect was persuasive enough to initiate the steps that led finally to the Manhattan Project and the development of atomic bombs.
Here's Einstein's letter below:
Here's Roosevelt's response:
It was less than a generation later that America was locked in nuclear struggle with post-war Soviet Union.
Then, four or fives generations pass and now there are several countries with nuclear bombs.
Worst of all, the West finds itself locked into negotiations with Iran over uranium enrichment.
Less than a century has passed since Einstein and Szilárd expressed concern about the Germans, and the situation is no less complicated and no less dire.