Why did Julius Cesar invade Britain?

Through increased trade, Britain was developing closer links to the European continent. Moreover, the various British tribes of Southern England provided men and materiel for their counterparts in Gaul and Belgae (modern-day France and Belgium) in their never-ending conflict with Rome. On a strategic level, therefore, Britain was perceived by Caesar as a threat to his plan to pacify and subdue Gaul. Invading and successfully conquering the island would effectively cut off a crucial line of support to the Gallic tribes, making it easier for the Romans to defeat them.
The first invasion of Britain in 55 BC was more of a reconnaissance mission, designed to staunch the flow of men and arms making its way across the English Channel. The second invasion, in 54 BC, was much more ambitious, and though Caesar never actually conquered Britain, he left the island under Roman control by installing a British puppet ruler, the chieftain warrior Mandubracius.