Final answer:
Gaius Julius Caesar was the Roman leader who invaded Britain and Germany in 55 B.C. and spent years in Gaul, conquering the land and altering the course of European history with his military feats and cultural assimilation.
Step-by-step explanation:
The Roman leader who invaded Britain and Germany in 55 B.C. and spent several years in Gaul was Gaius Julius Caesar. Caesar, a renowned Roman general, statesman, consul, and author of Latin prose, initiated the campaigns that would lead to the expansion of the Roman Republic into a vast empire. During his campaigns, Caesar not only crossed the Rhine River, building a bridge in the process, but he also conducted the first Roman invasion of Britain. From 58 to 50 BCE, Caesar waged a long and brutal campaign in Gaul, during which he engaged in significant battles, enslaved numerous captives, and solidified his military reputation.
His conquests in Gaul were extensive and harsh. By the end of these campaigns, through his military prowess and written accounts, Caesar had thoroughly conquered the lands, ensuring that the impacted cultures would speak languages influenced by Latin for many generations to come. This level of conquests and cultural assimilation epitomizes the Roman reach into Europe under Caesar's command. His involvement in the First Triumvirate with Pompey and Crassus and his subsequent military actions dramatically shaped the history of Rome and its territories.