At the end of WWI, The Treaty of Versailles made Germany pay war reparations to repair & rebuild France & Belgium with the amount of money (£6.6 billion) only being officially decided on in 1921. Because America never joined the League of Nations even though it was Wilson's idea (President Wilson was a democrat & the rest of congress were Republican), they then were perfectly able to go & help Germany pay off their reparations & so they did.

With the payment strategies being drawn up in the "Dawes Plan (1924)" & the "Young Plan (1929). During the 1920's America was in the middle of a massive economic boom due to Isolationism & the rise of import/export tariffs after WWI. Because America helped Germany pay their debts when America was booming so was Germany (think of "Weimar Berlin") however come 1929 when Wall Street crashed America fell & so did Germany.

When they fell into a great depression, the public were looking for any way out of this situation, they were against the government. So there' Hitler who says he can make Germany great again. People start to believe Hitler's their last hope ("unsere letzte hoffnung hitler")

Sorry this was a quick response I don't have alot of time & this isn't really my expertise, perhaps someone who specialises in 20th Century Germany could give a more in depth answer