6 ideas
The weaker side's war runs through three phases: a strategic defensive while the enemy advances, a strategic stalemate while the enemy consolidates and the defender builds strength, and a strategic counter-offensive once the balance has shifted. Knowing which phase you are in governs what to attempt and what to refuse.
Japan was militarily strong but small, short of manpower and resources, fighting an unjust war, and diplomatically isolated. China was militarily weak but large, populous, fighting a just war, and able to win outside support. Because each side's advantages and weaknesses point in opposite directions over time, neither quick subjugation nor quick victory is possible, so the war must be long, and the long war favors China.
A weak defender gives up territory deliberately rather than spending its forces holding ground. The invader then stretches its lines, garrisons more land, and thins its concentration. Each mile surrendered buys time to mobilize, while each mile occupied becomes a cost the attacker must keep paying.
The strategically weaker side should fight quick-decision offensive battles at the campaign and tactical level, even while its overall posture is protracted and defensive. It concentrates superior force against isolated enemy units, destroys them, and moves on. Many small, lopsided victories wear down a stronger enemy until the strategic balance reverses.
War is politics with bloodshed, and the deciding resource is the mass of the people, not weapons. Explaining the war's aims to peasants, soldiers, and allies turns the whole population into recruits, suppliers, and intelligence, which drowns the enemy in a hostile sea. Military operations are subordinate to, and depend on, this political work.
For the weaker side, a single battle that stakes everything is a trap. Losing it ends the war, and winning it rarely destroys a stronger enemy. The rule is to fight only engagements you are sure to win, and to decline battles that would stake the nation's fate. Preserving your forces takes priority over holding any position.