By far, the most notable achievement of Emperor Claudius (reigned 41 to 54 A.D. / C.E.) was the adding of the Island of Britannia (the Roman name for Britain) to the Roman Empire.
There were a number of factors that encouraged the Romans to conquer Britannia. Julius Caesar had conquered the enormous territory of Gaul (present day France) by 52 B.C. / B.C.E., which meant that Britannia was only a short distance, across the English Channel, from the borders of the Roman Empire. Indeed, Caesar himself had twice invaded Britannia in 55 and 54 B.C. / B.C.E., though, unlike the invasion ordered by Claudius, Caesar's campaigns did not bring any portion of Britannia under permanent Roman rule.
The reason that Caesar had chosen to attack the Celtic tribes of Britannia was that these tribes were providing assistance to their fellow Celtic tribes of Gaul that Caesar was seeking to conquer. With Britannia left unconquered, the potential threat remained of its Celtic tribes interfering with the Roman rule of Gaul. The lure of wealth was another motivating factor for Emperor Claudius' invasion. Britannia contained valuable mineral resources, notable tin which was necessary for producing the metal known as bronze.
When a British king, who had lost his throne to a usurper and had fled to Roman Gaul, appealed to the Romans to help him regain his kingdom, Emperor Claudius had his excuse for invading Britannia. Thus, in the year 43 B.C. / B.C.E., he ordered a force of four Legions under the command of Aulus Plautius to begin the conquest of Britannia. This force crossed the English Channel and landed in the south of Britannia, defeating numerous Celtic tribal armies. Remarkably, Emperor Claudius himself arrived in Britannia with reinforcements for Aulus Plautius, including elephants (having never seen elephants, they made a great impression on the British Celts).
The conquest of the entire island of Britannia was not completed during the reign of Emperor Claudius. Indeed, despite numerous campaigns by different Roman generals and emperors, the norther half of Caledonia (the Roman name for Scotland) remained free of Roman rule. Nevertheless, roughly three-quarters of the island of Britannia was made part of the Roman Empire, which was a consequence of Emperor Claudius' decision to launch an invasion in 43 A.D. / C.E.