Graves was criticised at the time of publication for referring to the German spears as "assegais," a Zulu word that entered the English language in relatively recent times. In the Author's Note, Graves recognises the controversy but justifies his choice of words, both to distinguish the light German spears from the heavier "javelins" used by Roman troops, and to distinguish the "savage" Germans from the "disciplined" legionaries. The latter point, of course, seems as anachronistic to modern readers as anything from the pages of Suetonius or Tacitus.
Nobody knows what on earth did happen! The account here is elaborated from passages in Suetonius's The Twelve Caesars and Cassius Dio's Roman History, both of which refer to an abortive invasion attempt on Britain, which saw Caligula declaring war on Neptune instead of the British tribes.
During the course of these proceedings, Caligula seems to have accepted the "surrender" of the British chieftain, Adminius (one of the sons of Cunobelinus, King of the Catuvellauni). But it is more likely that Adminius was seeking refuge following a dynastic dispute within Britain.