Whenever the Quran encourages Jihad and expresses approval of it, while rebuking those who flinch from it and shirk it, cowardice is always the culprit. For the well-being of mankind, be it religious or worldly, cannot be complete without courage and generosity. The All-Glorious has explained that when someone turns his back on Jihad, God puts another in his place to perform it (Quran 9: 38-39, 47:38).
In courage and generosity for God's sake the greater merit belongs to those who take the lead (Quran 57:10). Courage does not reside in physical strength. A man may be physically strong yet faint at heart. Real courage is stoutness of heart and constancy. For fighting requires a body strong and fit for the task, but also a heart that is stout and skilful in battle. The commendable way to fight is with knowledge and understanding, not with the rash impetuosity or one who takes no thought and does not distinguish the laudable from the blameworthy. Therefore, the strong and valiant is he who controls himself when provoked to anger, and so does the right thing, whereas he who is carried away under provocation is neither courageous nor valiant.
"Public Duties in Islam" - Ibn Taymiyah, pp. 105-106