I’m pretty sure the only differences are that the US has different NOs.
Also, Russia is neutral, it has no units, it is not playable, and it cannot be invaded or have units non combat move into its territories.
US NOs:
When the United States Is at War:
� 30 IPCs if the United States controls Western United
States. Theme: Major shift from peacetime to wartime
economy.
� 5 IPCs if the United States controls both Alaska and
Mexico.
� 5 IPCs if the United States controls all of the following
territories: Aleutian Islands, Hawaiian Islands, Johnston
Island, and Line Islands. Theme: National sovereignty issues.
� 5 IPCs if the United States controls Philippines. Theme:
Center of American influence in Asia.
The link for the Pacific 1940 SE rule book:
http://www.wizards.com/avalonhill/rules/A&A_Pacific_1940_2ndEd_Rulebook_LR.pdf