وَيَوْمَئِذٍ يَفْرَحُ الْمُؤْمِنُونَ بِنَصْرِ اللَّـهِ (And on that day the believers will rejoice with Allah's help - Ar-Rum 30:4 ⧉). The wording of the text apparently implies that "Allah's help" in this verse Ar-Rum 30:4 ⧉ 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)
1. There is yet another explanation of this verse Ar-Rum 30:4 ⧉ offered by Sayyidna Ibn ` Abbas ؓ etc.They say that the news of the victory of Romans reached the Muslims the very day in which they defeated the pagans of Makkah in the battle of Badr. Therefore, the Muslims were delighted on their own victory when the news of the victory of the Romans added to their delight. According to this interpretation, "rejoicing of the believers with Allah's help" in the present verse Ar-Rum 30:4 ⧉ refers to the victory of the Muslims in the battle of Badr, and the good news given by this verse Ar-Rum 30:4 ⧉ is twofold; one, in relation to the Romans, and the other in relation to the Muslims themselves. (Muhammad Taqi ` Usmani)