I had an idea for how to reduce the silliness of having large tank armies in Yakutsk and Chinghai without completely banning them or making huge Mongolian armies appear out of nowhere due to “treaty violations.”
At the start of the combat move phase, all land units and air units must attempt to trace a line of supply to a friendly factory. Any given line of supply can run either by land or by sea, but not both.
A supply line running over land can stretch across up to 4 territories for infantry and/or artillery, and across up to 3 territories for all other units. This represents the greater difficulty of supplying fuel and spare parts to motorized units. The line must include your friendly factory, and the units you want to supply, as well as all territories in between them. For example, East Germany -> Poland -> East Poland -> Belarus would be a 4-territory supply line, so Germany can supply infantry in Belarus using the East German factory as long as Germany also controls Poland and East Poland. If the Allies have gained control of either half of Poland, or if Germany had tanks in Belarus instead of infantry, then the German units would have to trace some other supply line (e.g. to a German-controlled factory in Leningrad) or else be out of supply.
A supply line running over sea can stretch across up to 5 sea zones, and must run directly from the territory in which you have units to your factory. This means that units in an inland territory can never be supplied by sea, and it means that you may not shift over into an adjacent land territory in order to use its coastline. A sea zone with an enemy warship or submarine cannot be used to trace supply. Similarly, you may not trace supply through a canal (e.g. Suez Canal) unless you own all of the territories needed to pass through the canal.
Planes on carriers are automatically in supply, and all ships are also automatically in supply. Units in the same territory as your factory are likewise automatically in supply.
If you fail to trace a valid line of supply to a territory you control, you must place an “out of supply” marker on that territory. All units attacking out of a territory that is out of supply get -1 attack; this penalty applies even if the territory you are attacking would be in supply or would reconnect your supply lines. All units defending in a territory that is out of supply get -1 defense. All units moving out of a territory that is out of supply get -1 movement. If this would reduce a unit’s speed to 0, it can still move 1 space, but only during the non-combat phase. Similarly, if a territory is out of supply, units may only load onto transports from that territory or unload from transports into that territory during the non-combat phase.
A unit that leaves an out-of-supply territory and moves into a supplied territory will defend at its normal strength.
If you conquer a territory that was out of supply, immediately remove the out-of-supply marker; your opponent’s supply problems do not affect you. Likewise, if you conquer a territory that will be out of supply for you, do not add an out-of-supply marker yet – you do not have to add that marker until the start of your next combat phase.
At the start of your next combat phase, remove any out-of-supply markers in territories that are now receiving supply. All of the penalties from being out of supply are immediately cancelled. Of course, if the territory is still out of supply, then you should leave the out-of-supply marker in place. The only two times when you can adjust a territory’s supply are when you (a) conquer that territory, or (b) start your own combat phase. Otherwise, you should ignore any changes in your supply situation.