Why was Muhammad stealing? When the Apostle heard about Abu Sufyan coming from Syria, he summoned the Muslims and said, “This is the Quraish caravan containing their property. Go out to attack it, perhaps Allah will give it as a prey.” (Ibn Ishaq/Hisham 428)
In the books of Sīrah, it is clarified that this event took place during wartime. The Quraysh had gathered their wealth and collectively purchased the contents of Abū Sufyān’s caravan. The value of the caravan was substantial; moreover, it was transporting the weapons and supplies that they needed to invade Madinah and defeat the Muslims.
This caravan passed near to Madinah while en route to Makkah. And that is when the Prophet (upon him be peace) marched out with his companions to attack it. It is well known that Badr was a defensive battle in which the ill-equipped army of just over three-hundred men marched out under the white flag that was carried by Muṣʿab b. ʿUmayr to face a technologically superior and larger military consisting of the pagan Arabs lead by Abū Jahl.
The Battle of Badr is well known and the details surrounding it are discussed extensively. Qāḍī Ḥasan al-Mashshāt dedicated over one-hundred pages of his Inārat al-Dujā to this battle. Therein, he mentioned that Abū Sufyān’s caravan was the reason for the Muslims marching out.
Referring to this as “stealing” is far-fetched. Based on the established military protocols of then and now, a military intercepting an enemy’s arms shipment as a defensive measure is surely not considered to be “stealing.”
And Allāh knows best.
 Abū al-Ḥasan al-Nadwī, al-Sīrah al-Nabawiyyah, 299-300.
 Inārah al-Dujā fī Maghāzī Khayr al-Warā, 103-217.