The description on the Harry Potter wiki for Grindelwald says:
Grindelwald left Britain and soon stole the Elder Wand, building an army and rising to power in continental Europe. During his reign of terror, he murdered many wizards and Muggles.
The description for First Wizarding War says:
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.
Both of those descriptions sound the same. A Dark Wizard brings together a group of loyal followers and then terrorizes the wizard and muggle populations. Why isn't Grindelwald's conquest for power called the First Wizarding War?