Japan was encouraged by Great Britain & the USA to attack German colonies and holdings in China and the South Pacific. They also liked the way Japan handled Russia in 1905.
Japan thought it was reaching "equal status" with the "west"... etc.. They invested in a boom (bubble), the west started cutting them off from markets when it was apparent they were not willing to bow to European Superiority. The UK & USA feared their growing strength.
And... Japan was driven into a financial crisis resulting in nationalists gaining power. They promised to protect Japan from foreign intervention and to gain markets and resources by whatever means necessary.
Very simplistic explanation but read about the Showa Financial Crisis and Showa Era.
"One resists the invasion of armies; one does not resist the invasion of ideas" Victor Hugo
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