The Roman Empire has been defeated- (Quran 30:2)

The remarkable defeats of the Roman Empire under Heraclius. It was not merely isolated defeats; the Roman Empire lost most of its Asiatic territory and was hemmed in on all sides at its capital, Constantinople.

In a land close by; but they, (even) after (this) defeat of theirs, will soon be victorious- (Quran 30:3)

The defeat, "in a land close by" must refer to Syria and Palestine, Jerusalem was lost in 614-15 A.D., shortly before this Surah was revealed.
The Pagan Quraish of Makkah rejoiced at the overthrow of Rome by Persia. They were pro-Persian, and in their heart of hearts they hoped that the nascent movement of Islam, which at that time was, from a worldly point of view, very weak and helpless, would also collapse under their persecution.
But they misread the true Signs of the times. They are told here that they would soon be disillusioned in both their calculations, and it actually so happened at the battle of Issus in 622 (the year of Hijrah) and in 624, when Heraclius carried his campaign into the heart of Persia (see Appendix) and the Makkan Quraish were beaten off at Badr.

Within a few years. With Allah is the Decision, in the past and in the Future: on that Day shall the Believers rejoice- (Quran 30:4)

Bidh'un in the text means a short period-a period of from three to nine years. The period between the loss of Jerusalem (614-15) by the Romans and their victory at Issus (622) was seven years, and that to the penetration of Persia by Heraclius was nine years.
The battle of Badr (2 A.H. i.e. 624 A.D.) was a real time of rejoicing for the Believers and a time of disillusionment for the arrogant Quraish, who thought that they could crush the whole movement of Islam in Madinah as they had tried to do in Makkah. But they were signally repulsed.

With the help of Allah. He helps whom He will, and He is exalted in might, most merciful. (Quran 30:5)

(It is) the promise of Allah. Never does Allah depart from His promise: but most men understand not. (Quran 30:6)

The promise refers to the Decision of all things by the Command of Allah, Who will remove all troubles and difficulties from the path of His righteous Believers, and help them to rejoice over the success of their righteous Cause. This refers to all times and all situations. The righteous should not despair in their darkest moments, for Allah's help will come.

The prediction made in the initial verses of this Surah is one of the most outstanding evidences of the Quran's being the Word of Allah and the Holy Prophet Muhammad's being a true Messenger of Allah. Eight years before the Holy Prophet's advent as a Prophet the Byzantine Emperor Maurice was overthrown by Phocus, who captured the throne and became king. Phocus first got the Emperor's five sons executed in front of him, and then got the Emperor also killed and hung their heads in a thoroughfare in Constantinople. A few days after this he had the empress and her three daughters also put to death. The event provided Khusrau Parvez, the Sassanid king of Iran; a good moral excuse to attack Byzantium. For Emperor Maurice had been his benefactor; with his help he had got the throne of Iran. Therefore, he declared that he would avenge his godfather's and his children's murder upon Phocus, the usurper. So, he started war against Byzantium in 603 A. D. and within a few years, putting the Phocus armies to rout in succession, he reached Edessa (modern, Urfa) in Asia Minor, on the one hand, and Aleppo and Antioch in Syria, on the other. When the Byzantine ministers saw that Phocus could not save the country, they sought the African governor's help, who sent his son, Heraclius, to Constantinople with a strong fleet. Phocus was immediately deposed and Heraclius made emperor. He treated Phocus as he had treated Maurice. This happened in 610 A. D., the year the Holy Prophet was appointed to Prophethood.
The moral excuse for which Khusrau Parvez had started the war was no more valid after the deposition and death of Phocus. Had the object of his war really been to avenge the murder of his ally on Phocus for his cruelty, he would have come to terms with the new Emperor after the death of Phocus. But he continued the war, and gave it the color of a crusade between Zoroastrianism and Christianity. The sympathies of the Christian sects (i. e. Nestorians and Jacobians, etc.) which had been excommunicated by the Roman ecclesiastical authority and tyrannized for years also went with the Magian (Zoroastrian) invaders, and the Jews also joined hands with them; so much so that the number of the Jews who enlisted in Khusrau's army rose up to 26,000.
Heraclius could not stop this storm. The very first news that he received from the East after ascending the throne was that of the Iranian occupation of Antioch. After this Damascus fell in 613 A. D. Then in 614 A.D. the Iranians occupying Jerusalem played havoc with the Christian world. Ninety thousand Christians were massacred and the Holy Sepulcher was desecrated. The Original Cross on which, according to the Christian belief, Jesus had died was seized and carried to Mada'in. The chief priest Zacharia was taken prisoner and all the important churches of the city were destroyed. How puffed up was Khusrau Parvez at this victory can be judged from the letter that he wrote to Heraclius from Jerusalem. He wrote: "From Khusrau, the greatest of all gods, the master of the whole world:

To Heraclius, his most wretched and most stupid servant: You say that you have trust in your Lord. why didn't then your Lord save Jerusalem from me?"
Within a year after this victory the Iranian armies over-ran Jordan, Palestine and the whole of the Sinai Peninsula, and reached the frontiers of Egypt. In those very days another conflict of a far greater historical consequence was going on in Makkah. The believers in One God, under the leadership of the Prophet Muhammad (may Allah's peace be upon him), were fighting for their existence against the followers of shirk under the command of the chiefs of the Quraish, and the conflict had reached such a stage that in 615 A. D., a substantial number of the Muslims had to leave their homes and take refuge with the Christian kingdom of Habash, which was an ally of the Byzantine Empire. In those days the Sassanid victories against Byzantium were the talk of the town, and the pagans of Makkah were delighted and were taunting the Muslims to the effect: "Look the fire worshipers of Iran are winning victories and the Christian believers in Revelation and Prophethood are being routed everywhere. Likewise, we, the idol worshipers of Arabia, will exterminate you and your religion."
These were the conditions when this Surah of the Quran was sent down, and in it a prediction was made, saying: "The Romans have been vanquished in the neighboring land and within a few years after their defeat, they shall be victorious. And it will be the day when the believers will rejoice in the victory granted by Allah." It contained not one but two predictions: First, the Romans shall be victorious; and second, the Muslims also shall win a victory at the same time. Apparently, there was not a remote chance of the fulfillment of the either prediction in the next few years. On the one hand, there were a handful of the Muslims, who were being beaten and tortured in Makkah, and even till eight years after this prediction there appeared no chance of their victory and domination.
On the other, the Romans were losing more and more ground every next day. By 619 A. D. the whole of Egypt had passed into Sassanid hands and the Magian armies had reached as far as Tripoli. In Asia Minor they beat and pushed back the Romans to Bosporus, and in 617 A. D. they captured Chalcedon (modern, Kadikoy) just opposite Constantinople. The Emperor sent an envoy to Khusrau, praying that he was ready to have peace on any terms, but he replied, "I shall not give protection to the emperor until he is brought in chains before me and gives up obedience to his crucified god and adopts submission to the fire god."
At last, the Emperor became so depressed by defeat that he decided to leave Constantinople and shift to Carthage (modern, Tunis). In short, as the British historian Gibbon says, even seven to eight years after this prediction of the Quran, the conditions were such that no one could even imagine that the Byzantine Empire would ever gain an upper hand over Iran. Not to speak of gaining domination, no one could hope that the Empire, under the circumstances, would even survive. bid`i sinin, and the word bid` in Arabic applies to a number upto ten. Therefore, make the bet for ten years and increase the number of camels to a hundred." So, Hadrat Abu Bakr spoke to Ubayy again and bet a hundred camels for ten years.
In 622 A. D. as the Holy Prophet migrated to Madinah, the Emperor Heraclius set off quietly for Trabzon from Constantinople via the Black Sea and started preparations to attack Iran from rear. For this he asked the Church for money, and Pope Sergius lent him the Church collections on interest, in a bid to save Christianity from Zoroastrianism. Heraclius started his counter attack in 623 A. D. from Armenia. Next year, in 624 A. D., he entered Azerbaijan and destroyed Clorumia, the birthplace of Zoroaster, and ravaged the principal fire temple of Iran. Great are the powers of Allah, this was the very year when the Muslims achieved a decisive victory at Badr for the first time against the mushriks. Thus both the predictions made in Surah Rum were fulfilled simultaneously within the stipulated period of ten years.
The Byzantine forces continued to press the Iranians hard and in the decisive battle at Nineveh (627 A.D.) they dealt them the hardest blow. They captured the royal residence of Dastagerd, and then pressing forward reached right opposite to Ctesiphon, capital of Iran in those days. In 628 A. D. in an internal revolt, Khusrau Parvez was imprisoned and 18 of his sons were executed in front of him and a few days later he himself died in the prison. This was the year when the peace treaty of Hudaibiya was concluded, which the Quran has termed as "the supreme victory", and in this very year Khusrau's son, Qubad II, gave up all the occupied Roman territories, restored the True Cross and made peace with Byzantium. In 628 A. D., the Emperor himself went to Jerusalem to install the "Holy Cross" in its place, and in the same year the Holy Prophet entered Makkah for the first time after the Hijrah to perform the `Umra-tul-Qada'.
After this no one could have any doubt about the truth of the prophecy of the Quran, with the result that most of the Arab polytheists accepted Islam. The heirs of Ubayy bin Khalaf lost their bet and had to give a hundred camels to Hadrat Abu Bakr Siddiq. He took them before the Holy Prophet, who ordered that they be given away in charity, because the bet had been made at a time when gambling had not yet been forbidden by the Shari`ah; now it was forbidden. Therefore, the bet was allowed to be accepted from the belligerent disbelievers, but instruction given that it should be given away in charity and should not be brought in personal use.