Japan’s attack on Pearl Harbor signified not just Japanese aggression, as these attacks had been simmering and worsening for years, but Japanese 'escalation' of aggression. Even though rapid and deadly in their execution the attacks had been coming, albeit more slowly, for years. Surprisingly, the US had at one time viewed the Japanese and their ambitious plans with some sympathy, and even tolerated them up to a point. Badly hit by the depression of the 1930s though, the Japanese people's disillusion over party government developed and cautious moderates were overtaken by more militant thinkers. In 1931 the Japanese occupied Manchuria in north China. Over that decade conflict escalated and in the July of 1937 war was finally declared. As the discord intensified and illustrated its aggressive new approach, Japan's relationship with the US deteriorated.
They milked Manchuria dry of the resources they needed but didn't have themselves by establishing new light and heavy industry. This policy was seen as essential by the Japanese whose own country was natural resource poor, and their relentless search for new supplies underpinned their foreign,military and diplomatic policy right through the next decade and eventually led to the notorious attack on Pearl Harbor and the ensuing Pacific War.