Global wizarding war

This conflict was started by the Dark Wizard Gellert Grindelwald and his followers. The intentions of the conflict were to topple the European Ministries of Magic, replacing them with a Wizarding Empire that would control the entire wizarding population of earth, as well as enslaving the Muggle race.

Speculations

 * As Albus Dumbledore defeated Gellert Grindelwald in 1945, it has been thought by many people to mean that Grindelwald may have had a hand in, or may even have been the mastermind behind the infamous World War II. This would make sense, as it would be immensely easier to topple the Wizarding Governments if the Muggle World was too wrapped up in its own affairs to take notice of blatant magic.
 * In Harry Potter and the Deathly Hallows, Dumbledore mentions that he put off facing Grindelwald for some time before facing him. If the premise that Grindelwald is the mastermind behind World War II is taken to be true, then this statement by Dumbledore could mean that Grindelwald's plans were in motion from 1939, when the War began, and that Dumbledore put it off through that time.
 * It would be easy for him to start the war by placing the Imperious Curse upon several world leaders, or even just Adolf Hitler and the German Government.

Appearances

 * Harry Potter and the Deathly Hallows