As to Japans exact intentions I am not certain. What I do know is why Brigadier General William "Billy" Mitchell believed the attack would occur. After researching the effects of airpower versus battleships he saw the present weakness and predicted that Japanese expansionism would lead to conflict with the U. S. In the Pacific.

I personally believe that they saw the chance for a quick and devestating attack perhaps simply hoping for a few quick victories or to show us they were pissed for the help we were giving China.

Taken from national airforce museum factsheets.

During World War II, some of Billy Mitchell's warning came through, none more so than his famous prediction of war with Japan. In an official report submitted after his trip around the Pacific Ocean in 1924, Mitchell warned that Japan's expansionism would lead to conflict with the United States, and he foretold how a war would start. He stated that the war would begin with a surprise attack by Japanese forces on Pearl Harbor, Hawaii, in conjunction with an assault on the Philippines.

Attack will be launched as follows: Bombardment, attack to be made on Ford Island (in Pearl Harbor) at 7:30 a.m. ... Attack to be made on Clark Field (Philippines) at 10:40 a.m.

On Dec. 7, 1941, the Japanese attacked Pearl Harbor at 7:55 a.m. and Clark Field just hours later.