The conspiracy to provide Adolf Hitler with an Apple of Eden was not prevented by the Assassins. The Templars wanted to created a global conflict and then reconstruct the world with their vision : The New World Order. Winston Churchill, Joseph Stalin, Franklin D. Roosevelt were Templars, and created this plot.
The fact Assassins were not able to prevent it may be one of their biggest failures.
Following this failure and the worldwide war that would come, they had no choice but to remain hidden, acting only in the dark to ensure the world would not be rebuilt as the Templars wanted.
Letting go of the habits they have after World War I, they dropped off colored clothing and took a more military look.
An Interwar outfit
The White WWI coat
Obviously, the white long coats were completely inappropriate, and the color scheme for the ranks was changed.
In civilian clothing, the Novices In grey and dark red, the Assassins In black and red, the Master Assassins
The Assassin outfit
The Mentor wore a blue coat with an armband like the resistance fighter.
Master Assassin outfit
The novice would be given the Ring upon entering the order.
The mentor Armband
The Assassin ring
The assassins mostly helped the resistance fighter while trying to kill Hitler and the Apple back.
Eventually, Assassins were able to kill Hitler in his bunker, but the Templar Malfolter was able to retrieve the Apple and bring it to the current position of Grand Master Alexander Vidic.