Permission has been granted (to fight)…
أُذِنَ لِلَّذِينَ يُقَاتَلُونَ بِأَنَّهُمْ ظُلِمُوا وَإِنَّ اللَّهَ عَلَى نَصْرِهِمْ لَقَدِيرٌ. الَّذِينَ أُخْرِجُوا مِنْ دِيَارِهِمْ بِغَيْرِ حَقٍّ إِلَّا أَنْ يَقُولُوا رَبُّنَا اللَّهُ وَلَوْلَا دَفْعُ اللَّهِ النَّاسَ بَعْضَهُمْ بِبَعْضٍ لَهُدِّمَتْ صَوَامِعُ وَبِيَعٌ وَصَلَوَاتٌ وَمَسَاجِدُ يُذْكَرُ فِيهَا اسْمُ اللَّهِ كَثِيرًا وَلَيَنْصُرَنَّ اللَّهُ مَنْ يَنْصُرُهُ إِنَّ اللَّهَ لَقَوِيٌّ عَزِيزٌ
“Permission has been granted (to fight) to those who have been fought because they have been oppressed, and Allaah is All-Capable of aiding them. Those who have been expelled from their homes without right (for no reason) but that they say ‘Our Lord is Allaah’, and had Allaah not repelled some people with others then monasteries, churches, synagogues and Masaajid wherein the name of Allaah is mentioned abundantly, and Allaah will surely aid those who aid Him (i.e. His deen, Islaam), indeed Allaah is Powerful, Mighty.” – Surah Al-Hajj, Aayah 39-40.
This was the first Aayah revealed concerning Jihaad, prior to this there was no Jihaad, and the Muslims had to bear patience with the tortures carried out by the kuffaar, for thirteen long years in Makkah, then once Rasoolullaah Sallallaahu `Alayhi Wa Sallam was forced to migrate to Madeenah this Aayah was revealed.
This aayah and the ones that followed it concerning Jihaad abrogated any and all laws of Makkah with regards to having patience in the face of harassment.
Examples of what was changed:
1: If muslims are fought they must fight back, while previously they had to endure it patiently.
2: If Rasoolullaah Sallallaahu `Alayhi Wa Sallam is insulted or mocked then the insulter is to be killed, while previously Rasoolullaah Sallallaahu `Alayhi Wa Sallam had to endure it.
3: If the kuffaar merely intend to attack the muslims then they must be fought, while previously it wasn’t allowed.
4: Islaam is taken to other lands via Jihaad, while previously it wasn’t allowed.
5: If a hand span of Muslim land is invaded then the kuffaar must be fought, previously that wasn’t the case.
And there are similar other cases too.
From this Aayah we learn the way that the world’s wrongs are rectified, i.e. through Jihaad.
Many people try to gandhi-fy Islaam with their “Non-violence shall prevail!” opinions thinking that is the way forward.
But Allaah the Most-Knowledgeable says that the way forward is Jihaad.