First note that any unit which has tacticaly moved on your current turn will then be considered supplied until the start of your next turn. You can see this by the little supply barrels in the left corner of the unit pop-up and the green arrow indicateing tactical movement. So a stack that attacks this turn on the front will be considered supplied for defense on your opponents turn/s.
If any region is attacked by any unit (includeing air only) then that region will draw one supply from its reserve or a region adjacent to it, even if there are no units there. All of the units in that region are then considered supplied for additional attacks and will not draw any more supply.
Naval units (accept subs) that did not move on previous turn will draw one supply for defense from its zone or one adjacent if attacked but will not draw from an island in an adjacent zone. This also only uses 1 supply to cover all the units defense in that zone.
Also note that the Axis Player will need 1 additional supply to fight partisans in captured regions with population.
In particular, does an air-only attack cause expenditure of defensive supply?
If a given region is attacked multiple times, does a total of 1 supply cover it all? (I would assume so given the advice to keep 1 supply around near each region).
It seems clear that a point does not get expended if a region is attacked that has no units at all in it. Does a point of supply get expended if none of the units in a region defend? (eg an air attack vs infrastructure in an area with only infantry...or maybe in such a case infantry do defend, I forget)
It Still draws supply, although there seems to be no negetive effect of not having a supply there if only infantry are defending against infrastructure attack since the inf will not fire in that type of attack and the factory/res/rail sitll defend at 12 or 15 in winter.
I wonder if Japan could run the Chinese completely out of supply by air-striking multiple different regions each turn. They only get about 15 supply every 3 turns, so if you can hit more than 5 target regions each turn...
You can indeed try, but smart humans send lend lease supply to China, and the Chineese player will arrange his Flack/fighter to make that a risky adventure.
I have noticed that the free Chinesse militia created when they are attacked by land units do not need supply to fight. i.e. they never fight with supply penalties if supply is not available, but any other units there will have the penalty.