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Mandalorians are a culture (or group of cultures) in the Star Wars universe centered around the planet Mandalore. The Mandalorians have incorporated numerous species into their ranks and there is a lot of variation among them, but they famed for being a (mostly) warlike culture and for their highly distinctive armor, which is made of one of the few materials in the known universe that can resist lightsabers.
Talking about the history of Mandalorians is tricky because in addition to the Legends/Canon split, which is already contentious, you have the issue that a lot of the Legends lore about Mandalorians was invented by Karen Traviss, who is kind of the Matthew Ward of Star Wars (and Halo) novels - she absolutely loves the Mandalorians, since they push her "hard(ish) sci-fi badass military faction, yay!" love button, but she also loathes the Jedi and by extension all Force users, which often results in her turning the Mandalorians into the Star Wars equivalent of the Ultramarines.
The basic idea, boiling it down short, is that the Mandalorians are a warrior culture who arose in the far-flung past of the known galaxy. Whilst originally they were a distinct species, they were also an "assimilationist" type of empire-builder, striving to actively integrate conquered races into their culture by basically wiping out their old beliefs and making them follow the Mandalorian way. It technically paid off, because even though the Mandalorian species is long-extinct, the Mandalorian culture has remained in existence for thousands of years. Due to their warlike natures and ambitions of conquest, they have frequently fought against the Jedi, and in fact they usually allied themselves with the Sith in just about every one of the major Jedi/Sith wars - this is why they invented beskar, a mineral that can actually negate lightsaber energies, and made armor out of it.
Shortly before the time of the Clone Wars, there was an active civil war amongst the Mandalorians over whether or not their traditions of conquest and warmongering were worth keeping, considering it had consistently resulted in them getting the shit kicked out of them by the rest of the galaxy. This resulted in the pacifists temporarily taking over and booting the traditionalists off the homeworld, causing them to largely scatter across the galaxy as mercs and bounty hunters. Then the Clone Wars happened and a traditionalist terrorist group, the Death Watch, discredited the pacifists, assassinated their leader, and took over the planet again... which just led to more or less another civil war, resulting in Mandalore staying out of everybody's way during Palpatine's reign over the newly fledged Galactic Empire, save for the aforementioned legions of Mandalorian mercs and bounty hunters still roaming the galaxy and continuing fighting for money and kicks as if nothing had changed.
Mandos of note
- Jango Fett - Mandalorian bounty hunter who served as the genetic template for the Clone Troopers used by the Galactic Republic.
- Boba Fett - A clone of Jango that wasn't hyper-aged to serve as a disposable soldier, but instead given to Jango as a surrogate son as payment for his part in the Clone Troopers. Outlived his father and became a bounty hunter himself.
- Duchess Satine Kryze - Leader of Mandalore during the time of the Clone Wars, a representative of the pacifist movement that temporarily took control of the Mandalorians.
- Pre Vizsla - Leader of Death Watch, a Mandalorian terrorist group focused on forcibly overthrowing the pacifist government of their people.