What is the meaning of "Ghulibatir-Rum" (the Byzantines have been defeated) and "Adnal-Arḍ" (the nearest land) in Surat Ar-Rum, and what is the interpretation of "Wa hum mim ba‘di ghalabihim sayaghlibūn fī biḍ‘i sinīn" (And they, after their defeat, will overcome within a few years), and why will the believers rejoice on that day?

It is stated at the beginning of Surat Ar-Rum that the Romans were defeated by the Persians in the lowest land, but then they would defeat them within a few years. This indeed happened, and the believers rejoiced in Allah's victory, for several reasons: one being that the Romans were People of the Book while the Persians were idolaters, making the Romans closer to the Muslims. Also, the victory of the Romans was proof of the truthfulness of the Prophet's prophecy, peace and blessings be upon him. It was also said that the reason for the rejoicing was the victory of the Muslims over Quraysh, which coincided with the victory of the Romans over the Persians, either on the day of Badr or the day of Hudaybiyyah.