Al-Anfal: 72
{Those who emigrated}: Meaning, they departed from their homelands and their people out of love for Allah and His Messenger. They are the Muhajirun (Emigrants).
{And those who sheltered them in their homes and helped them against their enemies}: They are the Ansar (Helpers).
{They are allies of one another}: Meaning, they support one another in inheritance. The Muhajirun and the Ansar used to inherit from one another based on their emigration and support, excluding their blood relatives, until this was abrogated by the words of the Exalted: {And those of blood relationship are more entitled to one another}.
It is recited: (min wilayatihim), with both the fathah and the kasrah [on the 'waw'], meaning: from their mutual support in inheritance. The justification for the kasrah is that their mutual support is likened to a craft or trade; as if, by supporting his companion, he is practicing a task and engaging in work.
{Then it is your duty to help them}: It is incumbent upon you to support them against the polytheists.
{Except against a people}: Among them, {between whom and you there is a treaty}. It is not permissible for you to support them against those [with whom you have a treaty], because they do not initiate the fighting, as the covenant prevents that.
{And those who disbelieved are allies of one another. If you do not do this, there will be tribulation on earth and great corruption.}
