Actually I think the key is that, besides the center depots, we also need other supply depots.
They don't have to be "buildable", but there have to be some on the map acting as buffer hubs. Otherwise, we'll have to face nightmares like moving Russian own troops on its own far east territory with endless supply delays.:rofl:
Such things (the west coast of the USA too) are so obvious that I can't imagine the devs not doing anything about it or having a very good reason not to
If you start the game in 1936 with
all of your units in Washington, then they are all getting supplied directly from the supply centre for the "island" of continental USA, ie. Washington itself. Let's imagine you then strat. redeploy one Div. to Los Angeles (assuming S.R. will be possible in HOI3, it may not), and let's guess that this is 30 provinces away. Our Div. appears at it's new location and requires supplies for that day. For simplicity let's assume it requires 1 unit of supplies. It will use 1 unit from the supplies it carries with it. I've seen references of this being 30 days worth of supplies, but I'm not sure if this is speculation or this has come from the Devs. Personally I would have thought it would be more likely to be about 10 days, and 30 days only if put on offensive supply.
Having used 1 unit from it's own supplies the Div will issue a request to the province it is situated in to replenish this. The province is not able to do this, so passes the request on down the line, back to the supply depot at the capital. The capital then issues 1 unit of supplies to reach Los Angeles. Since these supplies travel one province per day, then it will take 30 days for them to reach Los Angeles. Just sufficient for them to reach the Div before it's own stock runs out.
Obviously, if the Div is further away than 30 provinces, or the supply stock carried by a Div is less than 30 days worth, then the Div will at some stage run out of supplies. But it doesn't matter (providing the Div doesn't engage in combat), the supply network will soon be formed and start to ensure that a flow of supplies is reaching the Div every day.
Now the first day, the Div wanted 1 unit of supplies and this message went back to the capital and 1 unit started its journey. The second day the Div will use up another 1 unit of supplies, and now will be 2 units down on it's stock level. It should request 2 units from the capital, and 2 units will start their journey. The third day, three units, etc. In this way, once supplies start reaching their destination province there is sufficient to meet the needs of the Div each day and to soon completely replenish its stock. Shortly the flow will reach an optimal level. There will be no supply base built in Los Angeles, but the Div will have it's own divisional supply train with a stock of 10-30 days worth of supplies. And there are always at least 30 units of supplies in transit from the capital to the Div.
Now this example is quite artificial. The idea that a Div will appear in a location 30 provinces away from the nearest province with any supplies is far-fetched. Let's assume a more realistic situation. There is a Corps with 3 Divs already located in the Los Angeles region on 1st January 1936. Between them they will have a considerable stock of supplies, and there will be at least 90 units of supplies already in transit from Washington to Los Angeles. If we S.R. another Div to Los Angeles the supply network doesn't need to be formed from scratch. It has to adjust to the greater demand from Los Angeles - 4 units per day, instead of 3 - but there is more than enough flexibility in the system to ensure that even a 33% increase in demand at this distance from the capital should not cause any of the Divs to run out of supplies.
If there is already an Army with 9 Divs in the Los Angeles area, and there is a Naval Base from which convoys are leaving to take supplies to the Pacific Fleet at Pearl Harbor and US Divs stationed in the Phillipines, then you can see that at any time there will be a considerable amount of supplies actually in Los Angeles, and also in transit from Washington to Los Angeles. The arrival of one more Div in Los Angeles in those circumstances would make little difference to the system, it will adjust itself with no significant impact on the Divs.
Clearly, if later in the war you feared an attack on the west coast from Japan, and redeployed a whole extra Army to the San Francisco area then the supply network would have some considerable amount of re-adjustment to make. At first, it might try to supply their needs from Los Angeles, before establishing another cross-country route direct from Washington to San Francisco. There might well be enough delay in this to cause some Divs either in LA or SF to run out of supplies, but it is still unlikely if they each carry 30 days worth. If this is the offensive supply level then it would make sense to give them this level before S.R.'ing them from Washington.
None of this would be an issue if the Divs arrive on the west coast by marching overland, instead of S.R. As they set out marching from Washington on their journey the supplies will just follow directly behind them. As long as they don't travel fast enough to move more than one province in a day (which I think even the fastest armoured/mechanised Divs wouldn't be able to achieve), then their supplies will always keep pace with them.
However, the system can't keep growing indefinitely. There is a limit, linked to the infrastructure in the provinces through which the supply flows. If this is exceeded then the system might not be able to adjust at all to deal with the increased demand, or (which seems more likely) it will take longer to adjust as an alternative route for the supplies is found. Either way, there is a greater likelihood that some Divs might run out of supplies every now and then.
But it is clear that there is little need to be able to build intermediate supply depots near the front. The Div and all of the provinces from the Div to the capital are in effect mini-supply depots.
