I heard that Rasulullah (P.B.U.H) migrated from Mecca to Medina (Hijra) as many muslims were being tortured and killed,to retaliate, he raided many caravans coming from Mecca, is this completely true, if so, why did the Prophet (SAW) do this?
The first verses of the Quran that were revealed about fighting are in Surah Hajj:
Permission (to fight) is given to those (believers) who are fought against, because they have been wronged; and surely, Allah is able to give them victory.
Those who have been expelled from their homes unjustly only because they said: "Our Lord is Allah.'' For had it not been that Allah checks one set of people by means of another, Sawami
, Biya, Salawat, and Masjids, wherein the Name of Allah is mentioned much, would surely have been pulled down. Verily, Allah will help those who help His (cause). Truly, Allah is All-Strong, All-Mighty.
From the translation of Tafseer Ibn Katheer:
Awfi reported that Ibn Abbas said, "This was revealed about Muhammad and his Companions, when they were expelled from Makkah.'' Mujahid, Ad-Dahhak and others among the Salaf, such as Ibn
Abbas, Urwah bin Az-Zubayr, Zayd bin Aslam, Muqatil bin Hayan, Qatadah and others said, "This is the first Ayah which was revealed about Jihad.''
Abbas said, "Then Allah revealed the words: (Permission (to fight) is given to those (believers) fought against, because they have been wronged; and surely, Allah is able to give them victory.)'' Abu Bakr, may Allah be pleased with him, said, "Then I knew that there would be fighting.'' Imam Ahmad added: "Ibn Abbas said, `This was the first Ayah to be revealed concerning fighting.''' This was also recorded by At-Tirmidhi and An-Nasa'i in the Book of Tafsir of their Sunans. At-Tirmidhi said: "It is a Hasan Hadith.''
The magazi, raids, of the Muslims in Medina began directly after this. These were military expeditions, in some cases as just a show of force, ordered by Muhammed peace be upon him. There are several such expeditions listed by Ibn Ishaq in his Sirah.
Ibn Ishaq further relates the events leading up to Badr, and how the prophet peace be upon him sent out an expedition to pursue the caravan of Abu Sufyan, and the other well known events surrounding the battle.
As for why, verse 40 of surah Hajj above:
"For had it not been that Allah checks one set of people by means of another..."
is similar to verse 251 of surah al Baqarah:
"And if it were not for Allah checking [some] people by means of others, the earth would have been corrupted, but Allah is full of bounty to the worlds."
Fighting, I.e. the conduct of war and fighting to defeat others with force of arms and other means is a general fact of human existence and civilisation, and Muslims were command to engage in it against others to prevent corruption and oppression, strengthen and defend the Muslims and to make Islam prevail over other religions.