《孙子兵法·虚实篇》
所以，着重防备前面，后方的兵力就薄弱；着重防备后面，前方的兵力就薄弱；着重防备左翼，右翼的兵力就薄弱；着重防备右翼，左翼的兵力就薄弱；处处防备，就处处兵力薄弱。
故备前则后寡，备后则前寡，备左则右寡，备右则左寡，无所不备，则无所不寡。
The reason why the enemy’s strength is weak is because it is so widely dispersed; the reason my strength is great is because I have compelled the enemy to divide its strength to guard me.