Islam

If the question is referring to the Rise of Islam period (620s-750), there are several key political and regional reasons why the Arab armies were capable of spreading as quickly and as successfully as it did. (It is worth noting that after this period, the Arabs were never again successful conquerors.)

1) Non-Organization of the Arab Tribes: When Mohammed himself was alive, he concentrated his efforts on uniting all of the Arab Tribes under one banner because they had historically been fragmented. As such, his ability to overtake "the next tribe" became easier due to previous successes. His first successes were based on converting the leaders of those tribes or entering into military alliances with them.

2) Sassanid-Byzantine Foreign Relations: The Sassanid Empire (of Persia) and the Byzantine Empire (of Turkey, the Levant, and Egypt) had considered the Arab Tribes to be cultural and political backwater as they had no civilization of their own nor any united military force which could pose a threat to their conventional armies. Thus, they saw each other as the only real powers in contention for control of Mesopotamia and they bloodied each other real well in this regard. This weakened their overall ability to resist invasions and attacks.

3) Peace After the Fall of the Sassanids: Being the weaker empire, the Arabs attacked Sassanid Persia. However, instead of rejecting their conquerors with long-term occupation, the citizenry was nullified. This is because a number of the Sahaba (or Apostles of Muhammed) were Persians themselves and were able to monitor the situation politically. This allowed the vast majority of Arab forces to turn West on the Byzantines instead of worrying about controlling Persia.

4) The Fall of the Byzantines: The Byzantine Empire was both outstretched and weak from its confrontations with the Sassanid Empire. The Arab forces (along with new Persian regiments) took them both by surprise and at weak, but strategic points. The Arabs did not press their luck into the hills of Anatolia, giving the Byzantine Empire terms for surrender.

5) Peace with the Amazigh (Berbers): West of the Byzantine Empire on the African Continent, the Amazigh Tribes saw no strong reason to oppose the Muslim forces since they had a much more massive force. Many Amazigh leaders converted and joined in the military conquests further westward. It is interesting to note that the majority of Muslims that invaded Spain (including their General al-Tariq - for whom Girbaltar is name) were Amazigh.

Answer 1

Mainly by the truthfulness of Islam religion, the correctness and Authenticity of Qur'an, the good Muslim model, and Islam morals.

Answer 2

Within the Islamic Empire, there were a number of conditions which facilitated the conversion of most Imperial Subjects to Islam after centuries of occupation. They worked together as a very strong system of carrots and sticks:

Carrots

1) Support of the Locals: Since the Byzantine Empire and Sassanian Empire were engaged in religious zealotry, it stands to reason that they did not tolerate their religious minorities very well. The Muslims professed to (and did) treat minorities far better than either of these Empires. This led to religious minorities (especially Jews) from within the conquered territories helping Muslims to secure outposts and positions, freeing the Muslims to continue to spread the religion.

2) Citizenship by Conversion: The Islamic Empire was interesting in that being the leader or a person of import in the Empire was not contingent on ethnicity or place of birth as was the case of most prior empires. All a person had to do to gain power and prestige was to convert to Islam. This simple integration mechanism resulted in massive number of Persians, Berbers, and Turks converting to Islam. In fact, the furthest expansions of Islam (Spain, India, West China, South Russia, the Balkans) were all accomplished by non-Arabs (respectively Berbers, Persians/Mughals, Turkmen, Azeris, and Ottomans).

3) Universality: Islam, as a religion, claims salvation for all people in all places, making the religion very attractive, especially to people born into low castes and bad economic positions. (This was especially helpful in India and saw numerous conversions of Sudras and Untouchables to Islam.)

4) Commerce: Islam also spread by means of commerce and education whereby non-Muslims learned of the faith and adopted as their own amidst business dealings with Muslims.

Sticks

1) Repression of the Non-Muslim Monotheists: The Dhimmi, or non-Muslim Monotheist under Muslim occupation was required to pay a number of taxes that were connected with his Dhimmi status. The most famous was the jizya, which was a tax that Dhimmi had to pay for Muslims for the right to not be killed where they stood for not acknowledging Mohammed's Prophecy; it was a form of humiliation. Additional taxes included the kharaj, which was a tax on non-Muslim land-holdings in the Muslim World. The kharaj was so untenable that most Dhimmi were forced to live in the cities where the tax would not be applicable. The above answer is also incorrect as concerns justice. On paper, a Christian or Jew could testify against a Muslim, but in reality, such testimony was not acceptable and the attempt to defame a Muslim would receive retribution. Christians and Jews were not allowed to build new houses of worship, restore old houses of worship, proselytize in any way (this included religious debate or dialogue), or allow wine or pigs to be shown in public.

2) Forcible Conversion of Polytheists: Polytheists were forced to convert to Islam with some rare exceptions (such as the Hindus in India since they were just too numerous and the territory was difficult to traverse). Zoroastrianism was the majority faith in Iran until Islam almost completely extinguished it, both by sword, economic inequality, and brutal repression of Zoroastrian customs (unless they could be Islamicized like Nourouz).

Abu Bakr was the father of Muhammad's (PBUH) Beloved wife Aisha.

Abu Bakr was two years younger than the Prophet. His parents named him Abdul Kaaba, which means the servant of the Kaaba. When he became a Muslim, the Prophet changed his pagan name to Abdullah. Howevr, in early youth he had adopted the surname of Abu Bakr. He had come to be known by this name among people. Even to this day, the world generally knows him as Abu Bakr.

The name of Abu Bakr's father was Uthman, but he was known as Abu Qahafa. Salma was Abu Bakr's mother. She was also known as Umm-ul-Khair. Abu Bakr belonged to a branch of the Quraish.

From early years, Abu Bakr was known for good and upright nature. He was honest and truthful. He came of a noble family. These things won him respect among the people. His goodness also won him the friendship of young Muhammad (Peace be Upon him). The two became fast friends in early boyhood. The friendship was to prove lifelong and history-making.

When he grew up, Abu Bakr became a rich merchang. But he used to be very kind-hearted. When he saw someone in trouble, his heart melted. He did his best to help him. If his money could remove suffering, he did not care home much he had to spend. Once he gave away thirty-five dirhams out of his total fortune of forty thousand. He was so honest in his dealings that people kept their money with him. Above all, Abu Bakr had a sincere heart and a firm will. Nothing could stop him from doing what he thought was the right thing to do.

These great qualities were soon to serve the noblest cause known to the world. Abu Bakr was to become the strongest supporter of the Redeemer of mankind. He was to become the first among the Companions. He was to make Arabia and thereby the world safe for Islam after the Prophet has passed away.

Answer:

The Shaikhayn, i.e. Hadrat Abû Bakr and Hadrat 'Umar 'radiy-Allâhu 'anhumâ', are the highest ones of the Ashâb-i-kirâm. Concomitant to a recent increase in the number of holders of bid'at, i.e. heretics, doubts have been being voiced concerning their superiority. So dreadful is the decaying trend that the correct tenets of belief taught by the Salaf as-sâlihîn (the early Islamic scholars) are being forgotten gradually. Indeed, it is an open fact based both on narrations and on logic that the Shaikhayn are the highest. Narrations come to us through three different courses. Allâhu ta'âlâ promised to His beloved Prophet in the fifty-fifth âyat of Nûr sûra that He would give him believing and pious Khalîfas and reinforce the Islamic religion through those Khalîfas. This fact is confirmed by the dreams which the Messenger of Allah had as well as by the dreams that the Ashâb-i-kirâm had and which the Messenger of Allah explained. Rasûlullah 'sall-Allâhu 'alaihi WA sallam' stated several times both directly and indirectly that the Shaikhayn would succeed him as his Khalîfas (caliphs) after him. His statements, which are documentary sources, have been conveyed to us through (an authentic chain of narrations and reports termed) tawâtur. Then, the Shaikhayn are the highest Muslims. It is stated in a hadîth-i-sherîf reported by Tirmuzî and Hâkim: "After me, follow Abû Bakr and 'Umar!" This hadîth-i-sherîf was reported by Huzayfa and ibn Mas'ûd. Hâkim's book quotes Enes bin Mâlik as having related: The tribe of Benî Mustalâq sent me to the Messenger of Allah to ask him to name the person to whom we were to pay our zakâts after him. When I came to Rasûlullah 'sall-Allâhu 'alaihi WA sallam' and asked him, he said, "Give them to Abû Bakr!" They sent me again. When I reported their question who would be the person to receive our zakâts after Abû Bakr, he said, "'Umar!" I came to him once again with the message asking for the name of the person to take our zakâts. The Prophet's answer was: "(You will be giving them to) 'Uthmân!" As the Messenger of Allah had to repair to bed during his last fatal illness, he appointed Hadrat Abû Bakr 'radiy-Allâhu ta'âlâ 'anh' as the imâm (to conduct the public prayers called salât, [or namâz,] in jamâ'at). He explicitly rejected the question if someone else could be the imâm. This was the event from which the notables of the Sahâba such as Hadrat 'Umar and Hadrat Alî inferred that Abû Bakr was to be (the first) Khalîfa. None of the Sahâba was opposed to their inference. According to a narration in Bukhârî, Abû Bakr as-Siddîq was conducting the morning prayer in jamâ'at with the command of Rasûlullah, when the blessed Messenger slightly raised the curtain hanging in the doorway and, seeing his Sahâba performing the namâz, he gave a happy smile. Thinking that the Messenger of Allah intended to come in and conduct the namâz, Abû Bakr as-Siddîq moved aside, which made the Sahâba rejoice with the same expectation. Motioning with his blessed hand, the most beautiful human being commanded, "Complete your namâz!" Then he let the curtain go down. He passed away that day. According to a narration unanimously reported by the scholars of Hadîth, one day a woman asked Rasûlullah a question. "Come back later and ask (the same question)," was the blessed Prophet's reply. The woman asked again, "O Messenger of Allah! What do I do if I can't find you here?" Rasûlullah stated, "If you can't find me when you come here, ask Abû Bakr!"

Abu Bakr:

Born to a wealthy family, Abu Bakr was a successful merchant with a reputation for honesty and kindness. Tradition has it that, having long been a friend to Muhammad(p.b.u.h), Abu Bakr immediately accepted him as a prophet and became the first adult male to convert to Islam. Muhammad married Abu Bakr's daughter Aishah and chose him to accompany him to Medina.

Shortly before his death, Muhammad asked Abu Bakr to offer up a prayer for the people. This was taken as a sign that the Prophet had chosen Abu Bakr to succeed him, and after Muhammad's death, Abu Bakr was accepted as the first "deputy of the Prophet of God," or caliph. Another faction preferred Muhammad's son-in-law Ali as caliph, but Ali eventually submitted, and Abu Bakr took over governance of all Muslim Arabs.

As Caliph, Abu Bakr brought all of central Arabia under Muslim control and was successful in spreading Islam further through conquest. He also saw to it that the Prophet's sayings were preserved in written form. The collection of sayings would be compiled into the Quran (or Q'uran or Koran).

Abu Bakr died in his sixties (between 60 & 69 years old), likely from natural causes. Before his death he named a successor, establishing a tradition of government by chosen successors .....And Uthman ibn Affan was chosen.

Hope this hepled :)