There are a lot of theories about what actually started the fire on the Hindenburg, an electrical short, lightning, sabotage, somebody smoking in the bathroom, but we don’t know for sure.
What we do know is that if the airship had been filled with nonflammable helium instead of hydrogen, the fire wouldn’t have been nearly as disastrous.
So why was it filled with hydrogen? Helium, a natural gas, isn’t common to most parts of the world, in fact, the United States has the vast bulk of the helium stores within its borders.
In the 1930s, the United States wouldn’t sell helium to the Nazis, so the Germans had to use the more dangerous gas, hydrogen, in their zeppelins.