It looks like the history of the Mandalorians goes a bit like this:
- Around 1000ish the Mandalorians are the marauding menace they were in Legends KotOR. They war with the Republic and the Jedi, sacking Coruscant at some point.
- The Republic eventually defeats them and Mandalore gets turned into a desert in the process. This crippling defeat and the near total destruction of the Mandalorians puts an end to their campaigns of conquest, although they remain a warrior people.
- By about 40 - 30 BBY Mandalore is in a state of cultural crisis: a movement called the New Mandalorians has risen which advocates non-violence (as seen in Rebels combat is a way of resolving disputes amongst Mandalorians and this presumably causes a lot of unnecessary injury and death.) There are also hardline factions that call for a full return of the old way of conquest where Mandalore is feared and takes whatever it wants by force.
- When Satine Kryze becomes the ruler of Mandalore she sympathises heavily with the New Mandalorians. Fearing the death of their culture the traditionalist factions attempt to remove her by force, starting the Mandalorian Civil War. Given their hostility to Death Watch the Protectors presumably are on the establishment side in this war.
- This civil war is so brutal it depopulates Mandalore and turns Satine, the New Mandalorians and the surviving populace full-blown pacifist. Those who refuse to embrace pacifism are sent into exile (presumably this is when the Protectors leave for Concord Dawn).
- The traditionalist faction survives as the Death Watch, led by Pre Vizla. He realises to take Mandalore he must win over the hearts and minds of the Mandalorian people as the New Mandalorians did: he must expose their military weakness and contrast it to the military strength of the old ways.
- Vizla's first attempt as shown in TCW is to aid the CIS in order to provoke a Republic invasion of Mandalore: he can then use the military occupation and historical hostility to the Republic to win the people over. His plans are thwarted by the Jedi.
- Vizla later finds Maul and the two form an alliance to take Mandalore and share power. Mandalore is pacifist and neutral: neither the Republic nor the CIS will aid it unless it takes a side in the Clone Wars, something Satine will never do. Maul is therefore able to ransack the capital, Sundari, with the military forces of the various crime syndicates he's gained influence over. Pre Vizla then "intervenes", defeats the syndicates, declares himself Manda'lor and double crosses Maul. The Death Watch becomes the ruling faction of Manda'lor with the support of the populace.
- Maul kills Vizla in single combat and declares himself Manda'lor. This causes a rift in the Death Watch: most recognise Maul's legitimacy (he killed Manda'lor and is therefore the strongest warrior in their eyes) but a significant number reject him as a non-Mandalorian outsider. The Death Watch splits into the Maul-loyal Supercommandos and the Vizla-loyal Nite Owls and Mandalore is once again in a state of civil war. Obi-Wan Kenobi is captured attempting to rescue Satine but escapes with the aid of the Nite Owls: they're willing to risk a Republic invasion to stop Maul.
- This time the state of Mandalore does trigger a Republic invasion: the Siege of Mandalore is one of the thus far untold stories and would have been the finale of TCW. It ends with Maul's defeat, Order 66, Rex and Ahsoka going into hiding and an Imperial occupation of Mandalore.
How to we get to where we are now?
As of the Siege Mandalore is ruled by Maul's Supercommandos with the Nite Owls being an insurgent force. With Maul's defeat by the Republic I'd wager the Supercommandos swore loyalty to the Empire (hence Imperial Supercommandos). The Empire as an expansionist dictatorship follows their ways (roughly), it defeated Maul and they've no issue swearing loyalty to an outsider that defeats their leader. This is heavily supported by the fact that the Imperial Supercommando leader, Gar Saxon, used to be one of Maul's Supercommandos.
The Nite Owls presumably become the "loyalists" mentioned by Gar Saxon in the trailer above: like the Protectors they believe that only Mandalorians can rule the Mandalorians although they're still probably mostly Death Watch in ideology. While Fenn Rau does serve the Empire initially he does it to keep the Empire out of Protectorate space: he joins the Rebels when his capture makes that no longer a possibility.
I can't find a bone to pick with that thorough assessment! Well done sir, you've either made Pablo Hidalgo proud or concerned that you may be a young upstart