Why did the Muslims rejoice at the Christians' victory in Soorah Ar-Rum? And did Allaah say that he helped the Christians?
All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
The Muslims rejoiced at the triumph of the Romans (Byzantines) over the Persians because the disbelievers of Quraysh and other Arabs, who were the enemies of Muslims, were idol worshipers. So the Muslims sympathized with the Romans in their war with the Persians given the fact that the Romans were Christians, People of the Book, while the disbelievers of Quraysh and other Arabs sympathized with the Persians, who were idol-worshipers like themselves, and wished for them to triumph over the Romans.
When the Persians triumphed over the Persians at first, the Muslims were saddened, then, Allah, The Exalted, revealed in the Quran that the Romans would defeat the Persians within a few years, and they did. Given the Companions' firm conviction in the divine revelation and trust in the promise of their Lord, Abu Bakr, may Allah be pleased with him, undertook a bet with one of the disbelievers that the Romans would defeat the Persians within a few years; this took place before betting was deemed prohibited. The promise of Allah was fulfilled and the Romans defeated the Persians.
It has not been stated in the Quran that Allah aided the Romans to triumph over the Persians. It is only stated that the Romans were to triumph over the Persians within a few years and that Allah grants victory to whomever He wills. Allah, The Exalted, says (what means): {Alif, Lam, Meem. The Byzantines have been defeated. In the nearest land, but they, after their defeat, will overcome, within three to nine years. To Allah belongs the command before and after. And that day the believers will rejoice, in the victory of Allah. He gives victory to whom He wills, and He is the Exalted in Might, the Merciful.} [Quran 30:1-5]
Allah knows best.
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