وَمَا جَعَلَهُ اللّهُ إِلاَّ بُشْرَى
Allah made it only as glad tidings. ..
Allah made sending down the angels and informing you of this fact as glad tidings,
وَلِتَطْمَيِنَّ بِهِ قُلُوبُكُمْ
and that your hearts be at rest therewith.
Surely, Allah is able to give you (O Muslims) victory over your enemies, and victory only comes from Him, without need to send the angels,
وَمَا النَّصْرُ إِلاَّ مِنْ عِندِ اللّهِ
And there is no victory except from Allah.
Allah said in another Ayah,
فَإِذَا لَقِيتُمُ الَّذِينَ كَفَرُواْ فَضَرْبَ الرِّقَابِ حَتَّى إِذَا أَثْخَنتُمُوهُمْ فَشُدُّواْ الْوَثَاقَ فَإِمَّا مَنًّا بَعْدُ وَإِمَّا فِدَاءً حَتَّى تَضَعَ الْحَرْبُ أَوْزَارَهَا ذَلِكَ وَلَوْ يَشَأءُ اللَّهُ لَانْتَصَرَ مِنْهُمْ وَلَـكِن لِّيَبْلُوَ بَعْضَكُمْ بِبَعْضٍ وَالَّذِينَ قُتِلُواْ فِى سَبِيلِ اللَّهِ فَلَن يُضِلَّ أَعْمَـلَهُمْ
سَيَهْدِيهِمْ وَيُصْلِحُ بَالَهُمْ
وَيُدْخِلُهُمُ الْجَنَّةَ عَرَّفَهَا لَهُمْ
So, when you meet (in fight in Allah's cause) those who disbelieve, smite (their) necks till when you have killed and wounded many of them, then bind a bond firmly (on them, take them as captives). Thereafter (is the time) either for generosity (free them without ransom), or ransom (according to what benefits Islam), until war lays down its burden. Thus, but if it had been Allah's will, He Himself could certainly have punished them (without you). But (He lets you fight) in order to test some of you with others.
But those who are killed in the way of Allah, He will never let their deeds be lost. He will guide them and set right their state. And admit them to Paradise which He has made known to them. (47;4-6)
and,
إِن يَمْسَسْكُمْ قَرْحٌ فَقَدْ مَسَّ الْقَوْمَ قَرْحٌ مِّثْلُهُ وَتِلْكَ الاٌّيَّامُ نُدَاوِلُهَا بَيْنَ النَّاسِ وَلِيَعْلَمَ اللَّهُ الَّذِينَ ءَامَنُواْ وَيَتَّخِذَ مِنكُمْ شُهَدَاءَ وَاللَّهُ لَا يُحِبُّ الظَّـلِمِينَ
وَلِيُمَحِّصَ اللَّهُ الَّذِينَ ءَامَنُواْ وَيَمْحَقَ الْكَـفِرِينَ
And so are the days (good and not so good), that We give to men by turns, that Allah may test those who believe, and that He may take martyrs from among you. And Allah likes not the wrongdoers.
And that Allah may test (or purify) the believers (from sins) and destroy the disbelievers. (3;140-141)
These are points of wisdom for which Allah has legislated performing Jihad, by the hands of the believers against the disbelievers. Allah used to destroy the previous nations that denied the Prophets, using various disasters that encompassed these rebellious nations. For instance, Allah destroyed the people of Nuh with the flood, `Ad with the wind, Thamud with the scream, the people of Lut with an earthquake and the people of Shu`ayb by the Day of the Shadow. After Allah sent Musa and destroyed his enemy Fir`awn and his soldiers by drowning, He sent down the Tawrah to him in which He legislated fighting against the disbelievers, and this legislation remained in the successive Laws.
Allah said,
وَلَقَدْ ءَاتَيْنَا مُوسَى الْكِتَـبَ مِن بَعْدِ مَأ أَهْلَكْنَا الْقُرُونَ الاٍّولَى بَصَأيِرَ
And indeed We gave Musa -- after We had destroyed the generations of old -- the Scripture as an enlightenment. (28;43)
It is more humiliating for the disbeliever and more comforting to the hearts of the faithful that the believers kill the disbelievers by their own hands. Allah said to the believers of this Ummah,
قَـتِلُوهُمْ يُعَذِّبْهُمُ اللَّهُ بِأَيْدِيكُمْ وَيُخْزِهِمْ وَيَنْصُرْكُمْ عَلَيْهِمْ وَيَشْفِ صُدُورَ قَوْمٍ مُّوْمِنِينَ
Fight against them so that Allah will punish them by your hands, and disgrace them, and give you victory over them, and heal the breasts of a believing people. (9;14)
This is why killing the disbelievers of Quraysh by the hand of their enemies, whom they used to despise, was more humiliating to the disbelievers and comforting to the hearts of the party of faith.
Abu Jahl, for instance, was killed in battle and this was more humiliating for him than dying in his bed, or from lightening, wind, or similar afflictions. Also, Abu Lahab died from a terrible disease (that caused him to stink) and none of his relatives could bear approaching him. They had to wash him with water by sprinkling it from a distance, then threw stones over his corpse, until it was buried under them!
Allah said next,
إِنَّ اللّهَ عَزِيزٌ
Verily, Allah is All-Mighty,
the might is His, His Messengers and the believers, both in this life and the Hereafter.
Allah said in another Ayah,
إِنَّا لَنَنصُرُ رُسُلَنَا وَالَّذِينَ ءَامَنُواْ فِى الْحَيَوةِ الدُّنْيَا وَيَوْمَ يَقُومُ الاٌّشْهَـدُ
We will indeed make victorious Our Messengers and those who believe, in this world's life and on the Day when the witnesses will stand forth (Day of Resurrection). (40;51)
Allah said next,
حَكِيمٌ
All-Wise.
in that He legislated fighting the disbeliever, even though He is able to destroy them and bring their demise by His will and power, all praise and honor is due to Him