Why was Japan's first step to expanding beyond China was to bomb the U.S Naval Fleet at Pearl Harbor?

Japan was an empire and World War II in the Pacific Theater was essentially imperial competition.  The British and French, as well as the Dutch had colonies and forces in Asia, and represented a threat to Japan's hegemony (dominance) in the region.  As Hitler conquered these places and threatened Britain to within an inch of its life, it became easier for Japan to take their holdings in Asia.

That meant the US was the only remaining unengaged, fully armed empire that could stand in their way.  While we had disagreements with Imperial Japan at the time, and tension was high, the simple and most compelling reason for them to attack us on December 7th was because we were the only ones left in their way.