Algeria, India, etc. all had profound independence movements manifest after WW2 that made them their own countries, but why specifically after WW2? Why not during the depression or other times? How did colonialism end so rapidly?
There were these moments in both World Wars where colonial troops were recruited to fight the wars in Europe. Here were non-European colonials, raised in a racial hierarchy that Europeans sat on top of, tasked with killing other Europeans. This created a kind of psychological break with the realities of the colonial hierarchy.
However, after WWI, France, Britain, Belgium, and the Netherlands were still powerful enough to forcefully maintain their colonies. So while there were independence movements in the colonies, they were generally limited to making demands for more colonial representation in their governments.
WWII changed all of this as Germany overran Belgium, the Netherlands, France, and continuously bombarded Britain throughout the course of the war. During and after WWII, these countries no longer had the resources to effectively maintain their colonies as they had before.
Adding to this was the radicalization of colonial troops who returned home after the war. Among those was the hugely influential post-colonial thinker [Frantz Fanon] (https://en.wikipedia.org/wiki/Frantz_Fanon). I can only remember a couple major points he brought up, but they were:
Nazism was the ultimate development of European colonialism. Whereas Europe conquered and colonized the rest of the world, Germany tried to conquer and colonize Europe. As such, the violence inherent in colonizing the rest of the world was brought back to Europe by Germany.
The establishment and maintenance of colonies was a violent process. Therefore violence could be expected in fighting for independence. The harder Europe tried to fight to keep its colonies, the more violent independence movements would be. Likewise, Europeans were willing to fight for liberation from Germany, so colonials should be willing to do the same against Europe.
There was a need to establish an independent national/social/personal identity distinct from Europe, and this could not be achieved with the domineering presence of Europeans (Gandhi shared a similar view, though advocated different methods).
Beyond crippling colonial infrastructure in, WWII also aroused colonial public consciousness, and both created the scenario for the rapid decline of colonies.