The reason why Muhammad bin Qasim Attacked Sindh.
Yaqut al Hamwi mentioned
وأن امرأة من المسلمين سبيت بالهند فنادت يا حجاجاه فاتصل به ذلك فجعل يقول:
A woman among the Muslims who was in captivity shouted O Hajjaajaah, When Hajjaaj was informed about it he said, Here I am, Here I am.
معجم البلدان – الحموي – ج ٥ – الصفحة ٣٥٠
As per Ahlulhadeeth historian of Pakistan, Muhammad Ishaq Bhatti,
Muhammad bin Qasim was Tabiee and in Basra he saw Anas bin Malik and others. He was not 17 rather he was 27 or 28 years of age when he came to India. Also, he had army of 20,000 when he came.
There were some Muslim rebels in India, Muhammad bin Harith Ullaafi and Muawiyah bin Harith Ullaafi who were taking help of Raja Dahir against Muslim Government. After the fall of Ullaafi rebellion, a Man Abdur Rahman bin Muhammad with the help of Rajas of Sindh, Makraan and Sajistan stood against Abdul Malik bin Marwan and Hajjaj bin Yusuf. And finally Sindhi Pirates attacked Muslims and when they were looting Muslim women. A woman shouted O Hajjaajaah, when someone informed about it to Hajjaj he said Labbaik. He wrote to Raja Dahir so that he may punish the pirates but Dahir refused. Hajjaj asked permission fro the caliphate to attack but it was refused but later on the permission was granted.
(Barre Sagheer me Islam k awwaleen nuqush by Muhammad Ishaq Bhatti under the biography of Ibn al Qasim).
Muhammad bin Qasim allowed Brahmins to do their worship freely.
Qadhi Muhammad Ismaeel bin Ali who was from the family of Musa bin Yaqub who was appointed by Muhammad bin Qasim as a qadhi of اروڑ, an elder from this family wrote Tareekh al Hind wal Sindh, Qadhi Ismaeel had scattered pages of this book. Ali bin Amir Abu Bakr saw these pages with him and translated in Farsi which is known as Chach Nama, it is mentioned in the book:
Muhammad bin Qasim said while giving them Amaan:
Make your temples.. Worship your idols as per your religion, make dealings with Muslims in your business, make corrections in your manners, help the poor brahmins.. Listen to your religious and political leaders, You have amaan.
(Chach Nama page 299, al Uqad at-Thameen page 155-156, Taken from Barre Sagheer me Islam k Awwaleen Nuqush by Historian Muhammad Ishaq Bhatti page 99)
The Son of Hasan Basri was a Mujahid in the army of Muhammad Bin Qasim when they came to India.
Khalifa bin Khyat said:
فحدثني ابْن كهمس بْن الحسن ، قَالَ : حَدَّثَنِي أَبِي ، قَالَ : ” كنت مع مُحَمَّد بْن القاسم فجاءنا داهر فِي جمع كثير ومعه سبعة وعشرون فيلا ، فعبرنا إليهم فهزمهم اللَّه وهرب داهر . قَالَ أَبِي : ثم عبرنا إليهم ، واتبع عصابة من المسلمين العدو فقتلوهم ثم رجعوا إِلَى العسكر ، فلما كَانَ فِي الليل أقبل داهر ومعه جمع كثير مصلتين ، فقتل داهر وعامة أصحابه ، وانهزم الآخرون ، واتبعهم مُحَمَّد بْن القاسم حَتَّى أتى مدينة برهما ، فخرج إليه قوم منهم فقاتلوهم فألجأهم إِلَى مدينتهم فحصرهم حَتَّى فتحها ، ثم سار إِلَى الكيرج فافتتحها “
The son of Kahmas bin al Hasan narrated from his father (Kahmas) that he was with Muhammad bin Qasim, (Raja) Dahir came with great army, he had 27 Elephants. Allah defeated them and Dahir fled.. Then Dahir came with his army at night but he and his men were killed… Muhammad bin Qasim followed (the remaining) men until they reached the city of Brahma.. (Tareekh Khalifa bin Khyat 236)
Jutt vs Thakurs.
Jutt (Cast of Indian subcontinent) sided Muhammad bin Qasim and Thakurs were the warriors of Raja Dahir.
As per the narration of al-Balādhurī when Muhammad bin Mus’ab (General of Muhammad bin Qasim) went to Muhammad bin Qasim from سدوسان (a city in maghrib of river of sindh) he had 4000 Jutt from سدوسان with him (who made treaty with Muslims.) (Futuh al Bildan page 425, taken from Barre Sagheer me Islam k awwaleen Nuqush by Historian Muhammad Ishaq Bhatti page 153)
As for Thakurs, Al-Baladhuri (279 h) said:
“ولقيه محمد والمسلمون وهو على فيل وحوله الفيلة ومعه التكاكره فاقتتلوا قتالا شديدا لم يسمع بمثله”
Muhammad (bin qasim) and Muslims met him (Raja Dahir) while he was over an elephant and there were other elephants around him, and Thakurs were with him (Raja). They fought furiously which was unheard of.
فتوح البلدان ص 422
صلاح الأمة في علو الهمة – ج 7 ص 91
People of Hyderabad welcomed Muhammad bin Qasim
Also people of Hyderabad (Sindh) welcomed Muhammad bin Qasim and Muslim army as they had a treaty with them. (Barre Sagheer me Islam k awwaleen Nuqush by Historian Muhammad Ishaq Bhatti page 151)
Muhammad bin Qasim was just ruler, so much that When he became captive, People of Hind (Hindus and others) cried.
فبكي أهل الهند على محمد وصوروه بالكبرج
People of Hind cried over Muhammad and made his picture in Kabraj. (Futuh al Bildan page 262)