Which was revealed in Makkah

بِسْمِ اللَّهِ الرَّحْمَـنِ الرَّحِيمِ

In the Name of Allah, the Most Gracious, the Most Merciful.

وَيَوْمَئِذٍ يَفْرَ‌حُ الْمُؤْمِنُونَ بِنَصْرِ‌ اللَّـهِ (And on that day the believers will rejoice with Allah's help - 30:4). The wording of the text apparently implies that "Allah's help" in this verse refers to His help for the people of Rum. Although they too were disbelievers, yet compared to others, their kufr was light. Therefore, their being helped by Allah Ta’ ala was not a far-fetched thing, especially when it was a matter of happiness for the Muslims.

The other possibility is that "Allah's help" in the above sentence means Allah's help for the Muslims, for which two explanations could be assigned. First, that Muslims had presented the victory of Rum as a sign of the truthfulness of the Holy Qur'an and the veracity of Islam. Hence, the victory of the Romans was in fact the victory of Muslims. The second reason for the help of Muslims could be that, there were two big powers of the disbelievers at that time - Persia and Rum. By making them fight with each other, Allah Ta’ ala made them weak, which helped the Muslims defeat them later. (Ruh).(1)