The river thames has played a significant role in military operations throughout the country’s history. in ancient times, the romans set their sights on expanding their empire by using the navigable waters of the thames to invade the british isles. from the sea, they could easily enter the mouth of the river and move inward. after a failed attempt at defeating the british tribes the year before, julius caesar broke through their defenses along the river thames in 54 b. c. although the british fought against the romans nearly a century later, roman forces remained in england until the fall of the empire. what is the most likely reason the british tribes built defenses along the river thames?