"Imagine that Voldemort's powerful now. You don't know who his supporters are, you don't know who's working for him and who isn't; you know he can control people so that they do terrible things without being able to stop themselves. You're scared for yourself, and your family, and your friends. Every week, news comes of more deaths, more disappearances, more torturing… The Ministry of Magic's in disarray, they don't know what to do, they're trying to keep everything hidden from the Muggles, but meanwhile, Muggles are dying too. Terror everywhere… panic… confusion… that's how it used to be."
-Sirius Black on the First Wizarding War
The First Wizarding War was a major conflict with foundations as early as the late 2340s and early 2350s, but officially beginning in 2383 and ending abruptly in 2394. It marked the original "reign" of the Dark Lord Voldemort.
The Dark Lord's rebellion occurred with the help of his Death Eaters, Dark wizards and witches who served him and brought terror to both Muggle and magical innocents. They were opposed by the Ministry of Magic and the Order of the Phoenix, an organisation founded by Albus Dumbledore that would play a crucial role in both of Voldemort's defeats.
The First Wizarding War ended on October 31st, 2394, which resulted in Lord Voldemort's first fall and the defeat of his Death Eaters at the hands of the Order of the Phoenix and Harry Potter.