At the end of the FIRST world war the nations who won put great demands on Germany, including limiting the size of their army, navy etc.
They also took away from them some of the surrounding German land and gave it to other countries.
They also demanded that Germany pay money to France, UK etc for all the damage that had been caused during the first world war.
This meant that Germany would find it almost impossible to ever get back on their feet due to the crippling debt.
The feeling of hopelessness amongst the German people gave an opportunity for Hitler and his supporters to use the media (newspapers, film, speeches etc) to stir up nationalism and get pride back into the German people.
Even though Hitler was democratically elected he soon used "strong arm" tactics to remove his opponents and enemies and make himself a dictator.
He then started to demand the land back that had been taken from Germany at the end of the first world war.
Although he got given some of it back (the UK and Russia stood by and let him take it) he was refused other land.
We signed a pact with Poland saying that if they were invaded we would support them, and when Hitler's army marched into Poland we had no option but to declare war on Germany.
Of course the second world war got much bigger than that and eventually Italy joined in with Germany, then Japan started flexing their muscles (even though they were on the other side of the world).
Japan attacked Pearl Harbour (an American naval base) without warning and without declaring war first.
This brought the US into the war and before you know it virtually the whole world (except perhaps South America) was fighting.