Why was Japan's attack on Pearl Harbor such a surprise?

Answer: It was a surprise because they gave no warning and at the time they were fighting China. And we weren't really that involved at the time.

The conventional wisdom held that the Japanese would try to seize the resources of Southeast Asia and perhaps attack the Philippines. Other analysts predicted an attack on the South Soviet Union. A Pearl Harbor attack was ruled out because it was believed the Japanese lacked the capacity to mount such an operation.