So they defeated them by permission of Allah, and David killed Goliath, (2:251)

﴿فَهَزَمُوهُم بِإِذْنِ اللَّهِ وَقَتَلَ دَاوُودُ جَالُوتَ وَآتَاهُ اللَّهُ الْمُلْكَ وَالْحِكْمَةَ وَعَلَّمَهُ مِمَّا يَشَاءُ ۗ وَلَوْلَا دَفْعُ اللَّهِ النَّاسَ بَعْضَهُم بِبَعْضٍ لَّفَسَدَتِ الْأَرْضُ وَلَٰكِنَّ اللَّهَ ذُو فَضْلٍ عَلَى الْعَالَمِينَ﴾
[ البقرة: 251]

2:251 So they defeated them by permission of Allah, and David killed Goliath, and Allah gave him the kingship and prophethood and taught him from that which He willed. And if it were not for Allah checking [some] people by means of others, the earth would have been corrupted, but Allah is full of bounty to the worlds.

They defeated them by God’s will.
The prophet of God, David, killed their leader Goliath and God gave him kingship and prophethood and taught him many sciences as He willed, including those that are beneficial for this world and those for the hearafter.
If it were not the system of God to drive back the evil of some people by means of others, there would be corruption on earth as the evil ones would gain authority.
However, God is Bountiful to all of His creation.

So they routed them by Allah's Leave and Dawud (David) killed Jalut (Goliath), and Allah gave him [Dawud (David)] the kingdom [after the death of Talut (Saul) and Samuel] and Al-Hikmah (Prophethood), and taught him of that which He willed. And if Allah did not check one set of people by means of another, the earth would indeed be full of mischief. But Allah is full of Bounty to the 'Alamin (mankind, jinns and all that exists).

phonetic Transliteration

Fahazamoohum biithni Allahi waqatala dawoodu jaloota waatahu Allahu almulka waalhikmata waAAallamahu mimma yashao walawla dafAAu Allahi alnnasa baAAdahum bibaAAdin lafasadati alardu walakinna Allaha thoo fadlin AAala alAAalameena

Abdullah Yusuf Ali - Translation

By Allah's will they routed them; and David slew Goliath; and Allah gave him power and wisdom and taught him whatever (else) He willed. And did not Allah Check one set of people by means of another, the earth would indeed be full of mischief: But Allah is full of bounty to all the worlds.

Safi-ur-Rahman al-Mubarakpuri

So they routed them by Allah's leave and Dawud (David) killed Jalut (Goliath), and Allah gave him (Dawud) the kingdom [after the death of Talut and Samuel] and Al-Hikmah (prophethood), and taught him of that which He willed. And if Allah did not check one set of people by means of another, the earth would indeed be full of mischief. But Allah is full of bounty to the `Alamin (mankind, Jinn and all that exists).

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