There are six major powers represented in SAS: Britain, the United States, Japan, Germany, France and Italy.
A campaign is played out between any two of these countries.
In a feature added in version 1.1, you can now also support the efforts of either side by nominating allies.
An ally is one of the six countries, currently non-aligned, who goes over to your side. An 'ally' is not therefore necessarily a country that in WW2 had an historical allegiance to the so-called 'allied' cause. The so-called 'axis' powers: Germany, Japan and Italy can also have their own allies.
An ally can contribute aircraft and ships of their own nationality. BUT - and this is a big but - they all must be paid for out of existing resources. You do not get any free lunch with allies. Just the ability to have their assets fighting in the war with you.
This can be advantageous especially with aircraft. Some nations simply had better aircraft; and the computer will automatically try to obtain the best aircraft possible, whenever the resources allow. (See how to build aircraft for more information.
With ships, there is also an advantage. Although you can already manually design and build the best ships you can think of, the ships built for you automatically (ie when your 2IC does this for you) are limited to those available (or those that might have been available) to your country in WW2. If you are a British player, you will need to manually design a Yamato-type battleship if you want one. But, if Japan were an ally, and you had the resources, your 2IC will try to 'obtain' a Yamato class ship whenever he judges that the balance of your naval ships requires such a big powerful battleship.
Allies can be added, and removed: