From: San Francisco
I figure things out by writing, even if some of what I write below is merely restating what others have written, and so...
I don't have the game in front of me, so this is just impressionistic: the RR from Paotow to Wuchang is Japan's greatest weapon. Japan can concentrate its forces at any number of places along that RR, and put China in the position of having to perhaps move reserves to shore up the attacked location and thereby weakening other points along the line, and then Japan can, if the first attack isn't to its satisfaction, move units to a new point along the line, possibly a newly weakened point, much more rapidly than China can (because of the RR).
Two points, then: First, what people have already alluded to, a move over the long desert trail to Lanchow represents Japan giving up this most powerful weapon, the ability to move quickly by making use of the RR as troops along that trail are far from the RR and far from their own reserves. In fact, from the point of view of China, the attack they might want to face is one through the desert to Lanchow. It seems the best defense for China in such an attack is to go after the lengthy and exposed lines of communication linking units attacking Lanchow and sources of supply far away. Indeed, a Japanese attack on Lanchow basically turns Japanese units into cousins of Chinese units: they will face a major concern about supply and will lack the benefits of mobility. And if these Japanese units give up the attack, it's a long walk back to the RR.
Second, China must use extreme economy in its use of ground units as moving many units to defend in one location weakens it elsewhere. While some Japanese attacks might require massive responses, say where a Japanese breakthrough is threatened at a key location, China must generally develop defensive strategies that make the optimal use of units that are close to the attack. True, sometimes units might have to be brought in from some distance, but the preferred Chinese solution is to throw as few units at a problem as possible. It seems the preferred solution is to get as much advanced notice of any Japanese offensive (by sending out trip-wire units and by closely reading the tea leaves, that is intel and combat reports) which would allow local reserves to have time to get where they need to be. The preferred defensive solution must also be premised, where possible, on going after Japanese communications. After all, a small unit sitting on the road supplying Japanese units, which helps degrade Japanese firepower, might be worth 3 times what it would be worth actually on the field of battle.
I'm just thinking out loud here just to see if any ideas I have, of a general perspective about China, hold water. I really know nothing about such matters. My goal is to find a strategy/set of tactics that allows China to hold out for one or two months longer than otherwise would be the case.
You are learning very quickly! Your insights on how detection, movement, and decision making interact are spot on! However, your ideas on the impact of supply lines don't really match up with reality in WitP. The game is very forgiving when it comes to LCU supply in large part due to experience in the Pacific theater but the result is that the Chinese experience is fairly different than from real life. In real life small Chinese units could certain disrupt Japanese supply lines which were prone to being overstretched, it's not really the case in this game. Only in very extreme situations will the Japanese struggle to get supply to front line units in China. Your ideas on supply are probably how it should be but the game just doesn't work that way.