Was the attack on Pearl Harbor "just?"

The attack on
Pearl Harbor was a surprise attack on the US naval base in Pearl Harbor, Hawaii.  Japan's
primary reason for attacking the US was to strike at US naval power so Japan could continue its
expansion to areas throughout the Pacific, such as the Philippines, Guam, and Midway.  In order
to throw the US off, the attack on Pearl Harbor was necessary.  The US also had cut off valuable
oil supply to the island nation, which damaged their war efforts.  

You could
argue that Japan's attack on Pearl Harbor was unjust because at the time of the attack the US
was in the process of peace negotiations with Japan.  The attack came without warning on a
Sunday morning.