An advantage to the Muslim army was from the countries they conquered most were badly ruled and highly taxed. Thus the people of the country welcomed the Muslims with open arms hoping for a better future. In a source an Arab offers terms of surrender. He offered the Syrians security of lives, property and buildings. And as long as they pay poll tax. The Syrian empire became under the Muslims control in the year 636. In Emesa (Syria) the Jews and Christians and sided with the Muslims. In Spain, the Jewish communities rose and revolted against their rulers. This might have happened because the Arabs were now organized and co-operated with the aim to spread Islam.
What appealed to the people, the poor in particular was that all people were equal before Allah. That appealed to the poor especially as the poor people at that time were probably mostly slaves.
The neighbouring Byzantine and Persian Empire were fighting since 6O3 to 6O9. That meant they both would have lost booty, good soldiers and equipment so by the time the Islamic Empire decided to conquer them the two Empires were exhausted of war.
The Arabs showed people that they do not want to fight unnecessarily in source ten and twelve. If their opposition would surrender and pay tax to the Islamic empire then they would be free and no fighting would take place.
The reason why people used to not take part in battle was they were scared of not returning alive from the battlefield. * Muhammad said that swords were the key to paradise and that whoever died fighting for Islam would go straight to Paradise. That must have taken the fear out of the soldier's heart. *Muhammad also had said that A day and night of fighting on the frontier (boundary) is better then a month of fasting and prayer He might've said that because he wanted more people to fight for Islam and that must have encouraged more people to participate in battles.
* Muhammad taught the people that there was no god but Allah and that he was the messenger of Allah. He made it easy to learn maybe because there were many illiterates and they wouldn't understand complex things.
A Source quotes Abu Bakr's rules. 'Do not betray or steal any of the booty. Do not practice treachery or mutilation. Do not kill a young child, old man or a woman.' It shows that the Arabs purpose was not to cause destruction or kill people but was purely to spread Islam. That was what made them so successful the fact that their aim was purely for spreading Islam and nothing else.
They used the money from the lands they conquered to pay for their armies causing more people to join the army and making them stronger then ever. A modern Muslim says he is happy and proud about the spread of Islam. Those two sentences are a result of the Arabs success over 1O25 years ago.
