When Imam Ali (a) sent an expedition of 3000 soldiers under Ma'qil bin Qays Riyahi against the Syrians, he issued the following instructions.
Always keep the fear of Allah in your mind. Remember that you have to meet Him one day (let the fear of Allah guide you in all your activities against man) and your end will be towards Him and towards none else.
Do not fight against anybody unless he wishes to fight against you. During winters travel in the mornings and give your army a rest in the afternoons. Do rush through journeys (unless absolutely necessary). Travel by easy stages, and do not tire out your army during the journey. Do not travel during the early part of the evening because Allah has meant this to be time for rest and comfort and not for march and exertion, make use of these hours to give rest to your body and mind.
When you have rested then begin your march with trust and faith in Allah in the early hours of the morning.
When you face your enemy, stand in the midst of your army, never alone. Do not be over-anxious to fight and do not behave as if you craving for a combat or aspiring for an encounter, but at the same time do not try to avoid your enemy or to evade an engagement as if you are afraid or nervous. Keep my orders in mind and act accordingly until you get further instructions. Do not let the hatred and enmity of your opponents force you to a combat, do not begin a battle even if the enemy so desires unless you have explored every avenue of amity and good-will and have exhausted all the chances of a peaceful settlement.