The Quran

Preface

إِنَّ الَّذينَ آمَنوا وَهاجَروا وَجاهَدوا بِأَموالِهِم وَأَنفُسِهِم في سَبيلِ اللَّهِ وَالَّذينَ آوَوا وَنَصَروا أُولٰئِكَ بَعضُهُم أَولِياءُ بَعضٍ ۚ وَالَّذينَ آمَنوا وَلَم يُهاجِروا ما لَكُم مِن وَلايَتِهِم مِن شَيءٍ حَتّىٰ يُهاجِروا ۚ وَإِنِ استَنصَروكُم فِي الدّينِ فَعَلَيكُمُ النَّصرُ إِلّا عَلىٰ قَومٍ بَينَكُم وَبَينَهُم ميثاقٌ ۗ وَاللَّهُ بِما تَعمَلونَ بَصيرٌ

Yusuf Ali

Those who believed, and adopted exile, and fought for the Faith, with their property and their persons, in the cause of God, as well as those who gave (them) asylum¹ and aid,- these are (all) friends and protectors, one of another. As to those who believed but came not into exile,² ye owe no duty of protection to them until they come into exile; but if they seek your aid in religion, it is your duty to help them, except against a people with whom ye have a treaty of mutual alliance.³ And (remember) God seeth all that ye do.

The reference is to the Muhājirin and Anṣār, the Emigrants and the Helpers, the people who forsook their homes and adopted voluntary exile from Meccan company with their beloved Leader, and their good friends in Medīna, who gave them asylum and every kind of assistance, moral and material. Under the magnetic personality of the Holy Prophet these two groups became like bloodbrothers, and they were so treated in matters of inheritance during the period when they were cut off from their kith and kin.
The Believers (Muslims) were entitled to all assistance in matters of religion.
But if they were not strong enough to suffer voluntary exile on behalf of the Cause and make the personal sacrifices which their more ardent brethren in faith made, they could not reasonably ask for political or military assistance or protection.
If a community suffers voluntary exile on account of persecution and oppression, and some of its weaker brethren stay behind, holding fast to faith but not prepared for the higher sacrifice, the exiles have still a duty to help their weaker brethren in matters of religion. The exiles, being at open war against the state which oppressed them, would be free to fight against such state. But if the weaker brethren are in a state in mutual alliance with the community, the community cannot in honour interfere with that state, whether it is Muslim or not. Presumably the alliance implies that the grievances of the weaker brethren will be redressed by the state itself. But it is not honourable to embarrass your own ally.