Germany rebuilds its military which directly violated the Treaty. This worried surrounding countries as they did not wish to further any type of conflict. This marks the start of WWII.
Germany Invaded Poland
Noticing a common goal among themselves; Germany, Italy, and Japan (As well as some other smaller countries) joined forces or rather formed a pact to fight for domination of different parts of the world. They figured that together they would have no problem taking control.
France submits to avoid fighting
Germany wanted to bring in more German speaking individuals and as such invaded Austria which welcomed it. Czechoslovakia however didn't really welcome it but at the same time, could not refuse.
The US joins in WWII
Germany wanted to regain its lost territory in Poland. Germany soon invaded it to fight and establish dominance of once again.
France being surrounded and overwhelmed by enemy forces had no choice as they would either submit or be wiped off the map. They submitted to control.
After a surprise attack in Pearl harbor by Japan, The US could no longer remain out of the war. American blood had been spilled and the US would not stand for it.