Between 55 BC and the 40s AD, the status quo of tribute, hostages, and client states without direct military occupation, begun by Caesar's invasions of Britain, largely remained intact. Augustus prepared invasions in 34 BC, 27 BC and 25 BC. The first and third were called off due to revolts elsewhere in the empire, the second because the Britons seemed ready to come to terms. According to Augustus's Res Gestae, two British kings, Dubnovellaunus and Tincomarus, fled to Rome as suppliants during his reign, and Strabo's Geography, written during this period, says Britain paid more in customs and duties than could be raised by taxation if the island were conquered.
By the 40s AD, the political situation within Britain was apparently in ferment. The Catuvellauni had displaced the Trinovantes as the most powerful kingdom in south-eastern Britain, taking over the former Trinovantian capital of Camulodunum (Colchester), and were pressing their neighbours the Atrebates, ruled by the descendants of Julius Caesar's former ally Commius.
Caligula may have planned a campaign against the Britons in 40, but its execution was unclear: according to Suetonius' The Twelve Caesars, he drew up his troops in battle formation facing the English Channel and, once his forces had become quite confused, ordered them to gather seashells, referring to them as "plunder from the ocean due to the Capitol and the Palace". Alternatively, he may have actually told hem to gather "huts", since the word musculi was also soldier's slang for engineer's huts and Caligula himself was very familiar with the Empire's soldiers. In any case this readied the troops and facilities that would make Claudius' invasion possible three years later. For example, Caligula built a lighthouse at Bononia (modern Boulogne-sur-Mer), the Tour D'Ordre, that provided a model for the one built soon after at Dubris (Dover).
In 43, possibly by re-collecting Caligula's troops from 40, Claudius mounted an invasion force to re-instate Verica, an exiled king of the Atrebates.Aulus Plautius, a distinguished senator, was given overall charge of four legions, totalling about 20,000 men, plus about the same number of auxiliaries. The legions were:
The II Augusta is known to have been commanded by the future emperor Vespasian. Three other men of appropriate rank to command legions are known from the sources to have been involved in the invasion. Cassius Dio mentions Gnaeus Hosidius Geta, who probably led the IX Hispana, and Vespasian's brother Titus Flavius Sabinus the Younger. He wrote that Sabinus was Vespasian's lieutenant, but as Sabinus was the older brother and preceded Vespasian into public life, he could hardly have been a military tribune. Eutropius mentions Gnaeus Sentius Saturninus, although as a former consul he may have been too senior, and perhaps accompanied Claudius later.