Claudius unexpectedly became emperor after Gaius' murder on Jan. 24, 41, when he was discovered trembling in the palace by a soldier. The Praetorian Guards, the imperial household troops, made him emperor on January 25. By family tradition and antiquarian inclinations, Claudius was in sympathy with the senatorial aristocracy; but soldiers and courtiers were his real supporters, while freedmen and foreigners had been his friends earlier. Initially, the attitude of the Senate was at best ambiguous. In 42 many senators supported the ill-fated rebellion of the Governor of Dalmatia. Even later, several attempts on Claudius' life involved senators and knights. Though paying homage to the dignity of the Senate (to whose administration he returned the provinces of Macedonia and Achaea) and giving new opportunities to the knights, Claudius was ruthless and occasionally cruel in his dealings with individual members of both orders. In some ways he was more like Julius Caesar, both in his willingness to admit non-Romans to the Senate and his wish to improve the efficiency of Roman government. In his dealings with the provinces, he favored a moderate extension of Roman citizenship by individual and collective grants: in Noricum, a district south of the Danube comprising what is now central Austria and parts of Bavaria, for instance, five communities became Roman municipalities. He encouraged urbanization and planted several colonies, for example, at Camulodunum and at Colonia Agrippinensis (modern Cologne) in Germany in 51.

Claudius tried, like Augustus, to work with the Senate and restore its honor and dignity. But the fact that he restored the office of censor (and thus removed and controlled who could remain a Senator) and added non-Romans like Gauls to it offended many Senators. The fact that Claudius took away Senatorial power through his administrative reforms (see below), and that he did, in the course of his administration, execute 35 senators (or so it is said), also helped alienate many Senators. But there are indications that, once Gaius had been killed, the idea of killing an Emperor became more imaginable, and several plots were hatched. Claudius, whose position as emperor was thus somewhat weak, probably overreacted at time to perceived threats and made unneeded killings.

In respect to administrative innovations, Claudius' general policy increased the control of the emperor over the treasury and the provincial administration and apparently gave jurisdiction in fiscal matters to his own governors in the senatorial provinces, in the name of increasing efficiency -- while taking power away from the Senate. He created a kind of cabinet of freedmen, head of organizational departments, for example Narcissus, who was chief of official correspondence (praepositus a libellis), Pallas, the secretary for finances (a rationibus), an overseer of petitions to the emperor (a libellis) and so forth. Again, the purpose of this was to create a more centralized, efficient administration of the empire, but in the long term his invention would become a growing cancer that would eat away at the resources of empire. The problem was that the officials in charge of these departments would use their powers also to enrich themselves, and, as the numbers of such officials grew (especially after the reorganization of the empire into many new administrative units under Diocleitian) their ever-increasing demands sapped the economic life of the empire.

Claudius also improved in detail the judicial system, especially in his measures that made trials move faster. But, he also often interfered by personally acting as judge in cases that should have been decided at a lower level, and often it seemed to many that his freedmen, who suggested to Claudius what cases to look into, were using the emperor to get at their enemies.

An impressive series of documents, such as a speech for the admission of Gauls to the Senate recorded on a partly defective inscription at Lugdunum (Lyon), the edict for the Anauni (an Alpine population who had usurped the rights of Roman citizenship and whom Claudius confirmed in these rights), and a letter to the city of Alexandria (41), survive as evidence of his personal style of government: pedantic, uninhibited, alternately humane and wrathful, and ultimately despotic. The inscription from Lugdunum is an interesting comparison with the version of the historian Tacitus in his Annals, which gives an account of the same speech. The speech as recorded in the inscription, in spite of irrelevance, inconsequence, and fondness for digression (much of which is absent in the version of Tacitus), shows that Claudius knew what he wanted and that he appreciated the latent forces of Roman tradition.

From the very beginning Claudius emphasized his friendship with the army and paid cash for his proclamation as emperor. Claudius' decision to invade Britain (43) and his personal appearance at the crossing of the Thames and the capture of Camulodunum (Colchester), were prompted by his need of popularity and glory. He, again like Julius Caesar, worked to the expansion of the empire. Claudius planted a colony of veterans at Camulodunum and established client-kingdoms to protect the frontiers of the province; these were afterward a source of trouble, such as the revolt in 47 of Prasutagus, client-king of the Iceni, and later the general revolt instigated by his wife Boudicca. He also annexed Mauretania (41-42) in North Africa, of which he made two provinces (Caesariensis in the east and Tingitana in the west), Lycia in Asia Minor (43), and Thrace (46). Though he enlarged the kingdom of Herod Agrippa I, he later made Judaea a province on Agrippa's death in 44. In 49 he annexed Iturea (northeastern Palestine) to the province of Syria. He was careful not to involve the empire in major wars with the Germans and the Parthians.

In his religious policy Claudius respected tradition; he revived old religious ceremonies, celebrated the festival of the Secular Games in 47 (three days and nights of games and sacrifice commemorating the 800th birthday of Rome), made himself a censor in 47, and extended in 49 the pomerium (the ritual border) of Rome. He protected the haruspices (diviners) and probably Romanized the cult of the Phrygian deity Attis. According to the biographer Suetonius in Claudius, during a period of troubles Claudius expelled the Jews from Rome for a short time; Christians may have been involved.

Claudius’ administration was also notable in its concern with public works, including the reorganization of the grain supply of Rome and construction of a new harbor at Ostia, which was later improved by the emperor Trajan. He completed two aqueducts badly needed in Rome, and roads in various parts of the empire.

The historical tradition treats Claudius rather harshly, as unduly influenced by his freedmen (who controlled access to him while enriching themselves and destroying their enemies) and by his wives. His wife Messalina was supposedly a prodigy of sexual activity; this marriage ended with her execution in 48, when she apparently conspired against him and, according to Tacitus, conducted a public marriage ceremony with her lover, Gaius Silius. Claudius married his niece Agrippina, an act contrary to Roman law, which he therefore changed. To satisfy Agrippina's lust for power, Claudius had to adopt her son Lucius Domitius Ahenobarbus (later the emperor Nero), to the disadvantage of his own son Britannicus. Roman tradition is unanimous in stating that Claudius was poisoned by Agrippina on Oct. 13, AD 54, though the details differ. A version of poisoning by mushrooms prevailed. Lucius Annaeus Seneca, the politician and satirist, who had been exiled by Claudius at his accession but had been recalled at Agrippina's urging to educate Nero, derided the dead emperor and his apotheosis (duly decreed by the Senate) in the satire Apocolocyntosis divi Claudii ("The Pumpkinification of the Divine Claudius"; the title and its exact meaning are both subject to dispute). The fact that he was so quickly made a god shows the real respect many had for him.