The Prophet (pbuh) himself, together with 200 men from the Muhajireen as well as Ansars, also led raids deep into enemy territory—and although they never engaged the enemy physically (at that time) this mere show of force drastically changed the perception the Qureish had of the Muslims.
Qureishi caravans travelling in near proximity to Medina were considerably beefed up with more camels and escorts. Seasoned warriors, armed to the hilt, kept careful watch over their precious cargo as it braved, not only the dangers of attackers, but the elements as well. Sandstorms and the murderous desert sun were known to decimate many an unsuspecting caravan.
All in all, Muhammed (pbuh) only sought to claim what was rightly theirs (the Muhajireen) as fair retribution for the belongings and properties they left behind in Mecca, or strike an agreement with Qureish to allow the Muslims (those who had families in the 'Holy City') free access into Mecca and to reclaim whatever belonged to them.
Qureish, on hearing this, did not only refuse outright, but spread malicious rumors about Muhammed's (pbuh) violation of the sanctity of the 'Holy Months' that all Meccans had to abide by, as well as his manifest evil in attacking their bread and butter.
And, as fate would have it, Abu Sufyan, the leader of Qureish at that time, was bringing a caravan through the desert that boasted an almighty amount of 2000 camels! The estimated value of merchandise it carried was 50,000 dinars! (1 dinar = 1 gold coin.)
So one can imagine, in today's terms, how much money that is!
Muhammed (pbuh) then gathered all his followers in Medina and prepared to attack the caravan while it was still near to the city. He knew that he would never get such an opportunity to strike at Qureish, where it would hurt them severely. He knew that the time was right for the first real confrontation of Islam!