Ibn Kathir
When the faithful party, who were few under the command of Talut, faced their enemy, who were many under the command of Jalut,
قَالُواْ رَبَّنَآ أَفْرِغْ عَلَيْنَا صَبْرًا
(they invoked: "Our Lord! Pour forth on us patience...") meaning, send down patience on us from You.
وَثَبِّتْ أَقْدَامَنَا
(and set firm our feet) meaning, against the enemy and save us from running away and from feebleness,
وَانصُرْنَا عَلَى الْقَوْمِ الْكَـفِرِينَ
(and make us victorious over the disbelieving people.)
Allah said:
فَهَزَمُوهُم بِإِذْنِ اللَّهِ
(So they routed them by Allah's leave) meaning, they defeated and overwhelmed them by Allah's aid and support. Then,
وَقَتَلَ دَاوُودُ جَالُوتَ
(and Dawud killed Jalut)
Israelite accounts claimed that (Prophet) David killed Goliath with a slingshot that he had, which he launched at Goliath causing his death.
Talut promised that whoever killed Jalut, would marry his daughter and would share his kingship and authority. He kept his promise. Later, the kingship was transferred to Prophet Dawud in addition to being granted Prophethood by Allah. So, Allah said:
وَآتَـهُ اللَّهُ الْمُلْكَ
(...and Allah gave him (Dawud) the kingdom) that Talut had and,
وَالْحِكْــمَةِ
(and Al-Hikmah) that comes with the prophethood, meaning, after Shamwil. Allah then said:
وَعَلَّمَهُ مِمَّا يَشَآءُ
(and taught him of that which He willed.) meaning, what He willed of the knowledge that He bestowed on (Prophet) Dawud.
Next, Allah said:
وَلَوْلاَ دَفْعُ اللَّهِ النَّاسَ بَعْضَهُم بِبَعْضٍ لَفَسَدَتِ الأَرْضُ
(And if Allah did not check one set of people by means of another, the earth would indeed be full of mischief.)
This Ayah indicates that if it were not for the fact that Allah checks one set of people with another, such as when Talut and the bravery of Dawud helped the Children of Israel (against Goliath), then people would have perished. Similarly, Allah said:
وَلَوْلاَ دَفْعُ اللَّهِ النَّاسَ بَعْضَهُمْ بِبَعْضٍ لَّهُدِّمَتْ صَوَمِعُ وَبِيَعٌ وَصَلَوَتٌ وَمَسَـجِدُ يُذْكَرُ فِيهَا اسمُ اللَّهِ كَثِيراً
(For had it not been that Allah checks one set of people by means of another, monasteries, churches, synagogues, and Masjids, wherein the Name of Allah is mentioned much, would surely, have been pulled down.) (22:40)
Allah then said:
وَلَـكِنَّ اللَّهَ ذُو فَضْلٍ عَلَى الْعَـلَمِينَ
(But Allah is full of bounty to the `Alamin (mankind, Jinn and all that exists)) meaning, by His mercy and favor He fixes some of them by some others. Surely, Allah has the wisdom, the supreme authority and the clear proof against His creation in all of His actions and statements.
Allah said:
تِلْكَ آيَـتُ اللَّهِ نَتْلُوهَا عَلَيْكَ بِالْحَقِّ وَإِنَّكَ لَمِنَ الْمُرْسَلِينَ
(These are the verses of Allah, We recite them to you (O Muhammad ) in truth, and surely, you are one of the Messengers (of Allah).)
This Ayah states, `These Ayat (verses) of Allah that We have narrated for you in truth conform to the exact manner that these stories have occurred and to the truth that still remain in the (Divine) Books that the scholars of the Children of Israel have and know. Allah said: O Muhammad,
وَأَنَّكَ
(you are)
لَمِنَ الْمُرْسَلِينَ
(one of the Messengers (of Allah)) emphatically stating the truth of his prophethood
المُيَسَّر
فهزموهم بإذن الله، وقتل داود -عليه السلام- جالوتَ قائدَ الجبابرة، وأعطى الله عز وجل داود بعد ذلك الملك والنبوة في بني إسرائيل، وعَلَّمه مما يشاء من العلوم. ولولا أن يدفع الله ببعض الناس -وهم أهل الطاعة له والإيمان به- بعضًا، وهم أهل المعصية لله والشرك به، لفسدت الأرض بغلبة الكفر، وتمكُّن الطغيان، وأهل المعاصي، ولكن الله ذو فضل على المخلوقين جميعًا.