Most people respond to aggression by in some way getting involved with it. It is almost impossible to hold back. By disengaging completely and retreating, you show great power and restraint. Your enemies are desperate for you to react; retreat infuriates and provokes them into further attack. So keep retreating, exchanging space for time. Stay calm and balanced. Let them take the land they want; like the Germans, lure them into a void of nonaction.
The Russians used this technique with Napoleon by not giving him a fight until he was deep in Russia with his army disheartened and his supply lines overstretched even destroyed.
