Going forth into Battle

Yet it is not necessary for all believers to march forth. Why not have some stay behind to gain deeper knowledge of religion and warn their people when they return, so they may be mindful?

9:122And the believers should not all go out to fight. Of every troop of them, a party only should go forth, that they (who are left behind) may gain sound knowledge in religion, and that they may warn their folk when they return to them, so that they may beware.

O you who believe, when you set out in the path of God, be discerning. Do not say to one who offers you peace, “You are not a believer,” driven by desire for the fleeting spoils of worldly life. With God lie abundant treasures far greater than these. You were once like them, then God graced you with His mercy. So be discerning.

4:94O ye who believe! When ye go forth (to fight) in the way of Allah, be careful to discriminate, and say not unto one who offereth you peace: "Thou art not a believer," seeking the chance profits of this life (so that ye may despoil him). With Allah are plenteous spoils. Even thus (as he now is) were ye before; but Allah hath since then been gracious unto you. Therefore take care to discriminate. Allah is ever Informed of what ye do.

We also sent iron, with its mighty strength and many benefits for humanity, so that God may test who will support His cause unseen.

57:25We verily sent Our messengers with clear proofs, and revealed with them the Scripture and the Balance, that mankind may observe right measure; and He revealed iron, wherein is mighty power and (many) uses for mankind, and that Allah may know him who helpeth Him and His messengers, though unseen. Lo! Allah is Strong, Almighty.