In my opinon, attack on pearl harbour was a big mistake.
US was waiting for a strong reason to enter world war on the side of the allies. Theorists say that few US leaders knew of the attack early on but chose to keep silent for obvious reasons. Japan could have tried negotiations with several other countries apart from US and waited for the outcome of German invasion of Russia before the tripartite pact between Germany-Italy-Japan, which was its also a big mistake. Their expansionist ambitions could have been better controlled if the army had been in check but the Empire lost it there as it could have been difficult to do so without the support of few cabinet
ministers.