Like gunpowder. there are a few other things that once you've smelled them you will likely never forget. One that I'll always remember and every once in a while, even today, sense the smell again is napalm. Naplam was first used during WWII mainly in flamethrowers but around 1964 a new formula, Naplam-B, was conceived to ignite more easily and to burn longer. During Vietnam over 388,000 tons of napalm was dropped over Indochina between 1963 and 1973.
Like the old line from the Apocalypse Now movie in 1979
“I love the smell of napalm in the morning, it smells like victory.”