Inna Allaha yuhibbu allatheena yuqatiloona fee sabeelihi saffan kaannahum bunyanun marsoos (surah Saff: 4)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(61:4) Allah indeed loves those who fight in His Way in ranks as though they were a solid wall. *3

Innal laaha yuhibbul lazeena yuqaatiloona fee sabeelihee saffan kaannahum bunyaanum marsoos

*3) First, it shows that only those believers are blessed with Allah's good pleasure and approval, who are prepared to fight in His way, disregarding every kind of danger. Secondly, that Allah likes an army which has three characteristics: (1) It fights in Allah's way with full understanding, and does not fight for a cause disapproved by Allah; (2) it is not indisciplincd, but is well-organized and well-arrayed for battle; and (3) it offers stiff resistance to the enemy as though it were "a solid cemented structure" . Then, this last quality is by itself highly meaningful. No army can stand in the battle array like a "solid structure unless it has the following characteristics.
(1) Complete agreement in the faith and the objective, which should smite soldiers and officers in a well-knit body.
(2) Confidence in one another sincerity, which cannot be created unless everybody is genuinely sincere in his aim and free from impure motives; otherwise the severe test of war will not allow anyone's insincerity to remain hidden, and whcn confidence is lost, the members of the army start suspecting instead of trusting one another.
(3) A high standard of morality without which neither can the officers and soldiers of the army have love and respect for each other, nor can they remain immune foam mutual clashes and conflicts.
(4) Love of the common objective and a firm resolve to achieve it, which should inspire, the whole army with an invincible spirit of gallantry, devotion and self sacrifice, so that it may actually face the enemy like a solid cemented structure in the battle field. Such were the foundations on which a powerful military organization was raised under the leadership of the Holy Prophet upon whom be Allah's peace and blessings). Mighty power of the day clashed with it and were annihilatcd; no power of the world then could dare face it for centuries.