Asalaamu alaikum [peace be upon you]

bro michael.
I was reading the biography of the Prophet Muhammad SAWS in the front of the Pickthall Qur'an translation. In it it talked about the Prophet's SAWS intention to capture the caravan led by Abu Sufyan returning from Syria. I have also read in other books about Islam that the Prophet SAWS used to raid caravans. This seems to me like nothing more than highway robbery, and that is very dishonest. I know that the Prophet SAWS was called Al-Amin, meaning "The trustworthy", so it seems completely out of character for him to lead raids on caravans.
Can anyone explain this for me? Plus the book also talks about the Prophet SAWS leading armies into battle, and the context that it seems to be in doesn't seem defensive, as I have been told fighting is permitted in.
Sorry for the late reply.
What happened is that there was a continuous warfare and threat from the Quraysh, and i assume you've read the Makkan era. Since many of the companions fled/emigrated [made hijrah] from Makkah empty handed, their wealth and houses were occupied and taken over by the disbelievers within Makkah.
When the believers emigrated to Madina, they were still chased after by Quraysh [as you'll read that when the muslims chase the caravan - the Quraysh are already sending an army to destroy and eradicate the muslims from Medina. This later becomes the battle of Badr] So why do they chase the caravans?
During warfare, many campaigns take place. Whether they are physical warfare, financial, and even of propaganda and talk. The Quraysh did this to the believers throughout their lives, and therefore muslims are permitted to do similar in an equal manner. The muslims weren't able to do that within Makkah, however - now that they are in Medina, they have to defend themselves and show that we are a worthy opponent. So they did similar to the Quraysh. Why is this justified?
1) To show their strength to the Quraysh aswell as surrounding tribes - so no-one messes with them, as many of the surrounding tribes (who were polytheists) were threatening muslims.
2) To get back their own wealth [which the Quraysh is now spending and using unjustly, and may even be using the muslims wealth in this caravan for their own benefit!]
3) It shows that it is permitted in warfare to get the enemy back, through different means [whether its through physical war, financially, aswell as doing other campaigns - the arabs media was poetry, so the Prophet, peace be upon him would get some companions (i.e. one of the best poets Hasan ibn Thaabit) to recite poetry in the muslims defence and support.)
Ever since the muslims reached Medina, the Quraysh and the surrounding tribes
[throughout that time - uptill the battle of Al-Ahzab/Khandaq (the confederates/trench) which was about 6 years after Hijrah] were threatening the muslim state. Compared to the event which you described which was only 2nd year after Hijrah (at this stage - the muslims were such a small minority compared to the rest of the polytheists in arabia who had respect for Quraysh and their forefathers religion - polytheism.) So it wouldn't be surprising if the muslims did many campaigns throughout this time to defend themselves aswell as harm the enemy.
You will also read campaigns which take place after the battle of ahzaab [after 6AH], and many of them will be when the muslims will leave Medinah and face the enemy head on, the reason for this is because the enemy has threatened them, or is preparing to fight the believers, so the believers reach them since this is known as the best form of defense - to surprise the enemy without them expecting it, which usually leads to them fleeing.
Putting fear into the heart of the enemy and making them flee is better. The believers are more pleased with less harm and more benefit.
Some useful links insha allah:
http://www.islamtoday.com/show_sub_section.cfm?main_cat_id=15&sub_cat_id=0
http://www.load-islam.com/artical_det.php?artical_id=414§ion=wel_islam&subsection=Misconceptions
http://www.islamonline.net/English/In_Depth/Violence/
Allah knows best.