Who was to blame for the trojan war? the greeks, the trojans, the gods or a specific individual?

Well it started when Eris, the goddess of discord, wasn't invited to a party (a wedding or something along those lines). In anger, she created a golden apple, barged into the party, and threw it into the center of the table, saying "this apple is for whoever is the fairest." To summarize, three goddesses fought over the right to the apple. They decided that they should have someone choose instead of just fighting over it. The man that picked who deserved the apple was mortal, and he chose Aphrodite because she promised him the most beautiful woman of all.
The mortal did get Helen, but she was actually the soon to be wife of a very important King. The King thought she was kidnapped and this huge war broke out for over 8 years over Helena, Queen of Troy.