I came up with an idea for one on one wars between two powers using the Global 1940 map. Here are the instructions for two versions (Soviet Union vs US and Germany vs Japan):
Common rules:
All other powers are neutral. No pro-Axis or pro-Allied neutrals are involved on either side. Attacking true neutrals is allowed with the usual effects of them siding with the other enemy of the attacker. Only territory of the belligerents (with exceptions detailed below) may be crossed.
For TripleA, use edit mode to make all powers neutral, including pro-Axis and pro-Allies.
An optional rule is to make pro-Axis and pro-Allied territories also true neutrals.
One side wins when all the units of the other side have been destroyed and all the territory of the enemy has been conquered.
Japan vs Germany:
To make the game faster, Trans-Jordan and Egypt will be under Japanese control and Gibraltar is under German rule.
All starting units and income is the same (Japan’s starting IPCs are still 26 even with the addition of Egypt and Trans-Jordan and the income effects of these two territories will only start at turn 2).
For TripleA, edit mode can be done to change the territories to their correct owners.
Additionally, Germany’s 10 IPC national objectives that are achieved (Iron Ore and Soviet trading) are kept, and no other national objectives are changed.
Japan’s trade with the US 10 IPC national objective is also kept.
For TripleA, edit mode is necessary for the US to be recorrected to be neutral as the US automatically declares war on turn 3.
Soviet Union vs US:
All starting units and income are the same.
The Soviets control Denmark, and like above the income change only applies starting at turn 2.
The US does not get their “at war” national objectives. This means that the US will only ever get 52 IPCs per turn until the US conquered Soviet territory.
Edit mode is required to make the US neutral with the Axis because the US automatically declares war on them at turn 3 for TripleA.
Soviet national objectives are the same. Due to not fighting the Axis, the Soviets will not achieve any. This intentional.
Remember to use edit mode to make all relationships between powers neutral.
Here are two test games I played on TripleA (TripleA is required to access these files) for both of these ideas. Keep in mind that since these were tests, what was done there may be inconsistent with what I outlined here, which are the definitive and final versions of the rules. Keep in mind I also used edit mode to change unit buys and to correct movement, but no moves or purchases were illegal. They might also give you some ideas of strategies.