WW1 was the most gruesome war up until that time. With the central powers fighting the allied powers, germany was defeated and forced to take all blame for WW1 which led to WW2. They were forced to admit the war was their fault, pay all damages, and lose their military.
After the war, all powers met in France and discussed the treaty of versailles and all allied powers except the U.S. made Germany sign the document and admit that the war was entirely their fault. They hoped it would keep Germany from starting another war but in reality it sparked them to get revenge.
Also in the treaty, the allied powers forced them to pay for all expenses in the war. The total cost was well over a billion dollars. This hurt the economic system of Germany and they couldn't pay for it all. It sparked the Germans to once again get revenge on the allied powers and gain control of their country again.
IN the treaty, the military power was ceased from Germany. They were now left defenseless. When Adolf Hitler comes to power the first thing he does is restore the army and navy so Germany isnt completely defenselss and he wants the strongest army in the world to go back out and defeat the allied powers.
Even though all the allied powers wanted was full revenge on Germany for their many expenses and casualities, it led to another war full of more expenses and casualities. Hitler stepped into power and didn't think it was fair for the Germans to be treated this way. WW2 then begins and it the most destructive war ever.