What political and economic challenges did the leading democracies face in the 1920s and 1930s?

Leading European democracies in addition to the US were challenged by crippled economies and devastated landscapes from World War I. Governments were wanting a return to more isolation and policies which favored their own countries. In the case of Germany, they were struggling to reestablish nationalism and were unable to defend themselves as a country. As a result many people in those countries turned to alternate forms of economy and government to create solid nations once again. All countries saw a rise in socialist thinking and some countries began to move toward socialist ideology. In others especially, Italy and Germany a move toward fascism took place which put nationalism at the forefront and a build up of capitalism to secure their economic place in the world. 

Very useful. it is to learn because history repeats itself so learning about it those mistakes not happen again.