After the Hijra (migration from Makkah to Madina) there
had been several small skirmishes fought between the Muslims
and the Quraish (clan to which the Prophet Muhammad 
belonged to) of Makkah. The first real large scale battle
between the Muslims and the Quraish was the Battle of Badr
which was fought on the 17th of Ramadan 2 AH (March 624 CE)
and took place at Badr, about 65 miles south of Madina.
This battle was of great importance in early Islam.
The intention of the Muslims was to intercept a trade caravan
travelling from Syria to Makkah which was led by Abu Sufyan.
The Muslims expected the caravan to stop at Badr (which was
on the trade route from Syria to Makkah) where they would
capture it.
Before reaching Badr, Abu Sufyan was informed that the Muslims
were intending to capture the trade caravan. He immediate sent
a messenger to Makkah to warn the Quraish. The Quraishi
leaders started to assemble an army to rescue the caravan.
As the Muslim army approached Badr, they learned of the news
that the Quraish had gathered an army which was on it's way
to Badr. Around this time Abu Sufyan changed course and
avoided the Muslim army.
The wells of Badr
The Muslims reached the valley of Badr before the Quraish and
captured the wells of Badr. The Prophet of Allah 
wanted to camp close to the nearest well. Hubab 
asked
if this was a divine order or whether it was the Prophet's
own opinion. When the Prophet of Allah
replied that it was his own decision, Hubab suggested
that they occupy the well closest to the enemy so that the
Quraishi army have no access to water.
The eve of the battle
On the eve of the battle Hazrat Muhammad wept and prayed
during the night and in the morning he led the Muslims in
prayer.
The Muslim Army versus The Quraishi Army
The Muslim army was commanded by the Messenger of
Allah himself which consisted of about 313 men, around
seventy camels and two horses. Prominent sahaba such as Hazrat
Hamza , Hazrat Abu Bakr , Hazrat Umar
and Hazrat Ali were also part of the army.
The army of the Quraish was commanded by Abu Jahl which
consisted of around 950 to 1000 men with around 700 camels
and about a 100 horses.
The Battle
The battle started when three warriors from the Quraishi army,
Utbah (father in-law of Abu Sufyan), Shaybah (Utbah's brother)
and Al Walid (Utbah's son) emerged and challenged the Muslims.
From the Muslim army three Ansaar of Madina emerged but the
Quraish refused to fight them. They started taunting the
Muslims and said that they have nothing against the people of
Madina and they only wish to fight the people from Makkah.
After this the Ansaar were called back and Hazrat Ali
and Hazrat Ubaydah , cousins of Hazrat Muhammad
and Hazrat Hamza , uncle of Muhammad responded
to the challenge. Hazrat Ali and Hazrat Hamza
killed their opponents but Hazrat Ubaydah , despite
killing his opponent, was severely injured and died a few days
after.
The Muslim army was ordered by Hazrat Muhammad to charge
the Quraishi army. It wasn't long before the Quraishi army tuned
their backs and ran, some were taken prisoner by the Muslims.
Abu Jahl the commander of the Quraishi army was killed along
with about 70 other Quraishi men. The Muslims lost about 14
sahaba that day, they were called the first shaheed (martyrs)
of Islam.
There is strong evidence in the Hadith and the Holy Quran that
Allah sent down angels led by Hazrat Jibraeel 
(Gabriel) to help the Muslims in the Battle of Badr. This was
the first major victory of Islam against the unbelievers.