I've had a chat with some of my buddies about the beginning of WW2 - specifically, when did it begin? We all knew that September 1st, 1939 was the 'official' beginning of the war, since this is the date most historians agree upon, but I was still not convinced.
Here are the number of options for you to choose from, and my explanations:
1.1931, September 18. Mukden Incident. Japan invades Manchuria and establishes a puppet state there [http://en.wikipedia.org/wiki/Mukden_Incident], Manchukuo [http://en.wikipedia.org/wiki/Manchukuo]. This may seem harmless, but this is the beginning of Japanese meddling in the Chinese remains of the Qing dynasty.
2. 1937, July 7-9. Marco Polo Bridge Incident [http://en.wikipedia.org/wiki/Marco_Polo_Bridge_Incident], which sparked the incursion of Japan into Chinese soil and the Sino-Japanese war which only ended in '45.
3. 1938, March 11 [http://en.wikipedia.org/wiki/Anschluss#1938], Annexation of Austria.
4. 1935, October [http://en.wikipedia.org/wiki/Second_Italo-Abyssinian_War], Italian-Ethiopian war.
5. Invasion of Poland by Germany, 1939, September 1. [http://en.wikipedia.org/wiki/Invasion_of_Poland]
6. Pearl Harbor bombing. 1941, 7-8 December. [http://en.wikipedia.org/wiki/Attack_on_Pearl_Harbor] 8th of December had the USA officially enter the war.
7. 1938, 30 September. [http://en.wikipedia.org/wiki/Munich_Agreement] The betrayal at Munich, where Czechoslovakia was royally screwed by its allies.
We had three dates in mind, but because I didn't want the thread to be about our discussion I also added 4 more dates to the mix. They may seem like the stepping stones towards WW2... you're probably right. Still, aggression can also be manifested in diplomacy.