It's safe to say that World War 2 could not have been prevented. Power-hungry nations were everywhere, bridled with the intent of conquering more land to expand their empires, and unwilling to relent in their pursuit for greatness.
With the idea of creating a new Roman Empire, Italy's Benito Mussolini led a plan which set in motion the invasion of Ethiopia in October 1935, Albania in early 1939, as well as a march into Greece shortly after.
Japan in it's own right had become a power-hungry nation as well. Their quest for domination, as it pertains to WWII, began with the invasion into the Republic of China in 1937.
Because of World War I, many nations lost a significant amount of land in their territory. Finland and Thailand are two such examples and in feeling a need to regain what they lost, joined in the war. Also in hopes of reclaiming what they felt was rightfully theirs, Hungary, Romania, and Bulgaria decided to align themselves with Germany, who had been their ally in the first world war. Despite Germany being a stronghold on its own, aligning with these countries helped Nazi Germany become an even stronger force.