Clitus became an officer of the Companion cavalry, a unit of eight squadrons (of 225 horsemen each) that was Macedonia's most effective weapon in battle. Its overall commander was Philotas, the son of Philip's most reliable general Parmenion. Clitus' exact position among the Companions is unclear, but he may already have been commander of the agema, the squadron that served as the king's bodyguard.
In any case, he was close to the young king during the battle of the Granicus, where Alexander defeated a Persian provincial army (June 334; text). During the fight, Clitus saved Alexander's life. He certainly was commander of the agema during the battle of Gaugamela (1 October 331).
He is next heard of in Susa, one of the capitals of the Achaemenid empire, where he fell ill. In 330, he brought Macedonian reinforcements to Parthia, where he joined forces with Alexander in September. By now, he was one of Alexander's most trusted officers, and he was the natural successor of Philotas when he was accused of treason in October 330 and executed. Clitus was probably not involved in the trial of his superior, which may have been masterminded by another officer, Craterus.
Alexander, however, had learned a lesson from the Philotas affair: it was dangerous to make one man sole commander of the Companions. Therefore, Clitus had to accept a second, minor commander, Alexander's closest friend Hephaestion.
The years 329 and 328 saw fighting in Bactria and Sogdia, satrapies in the northeast of the Achaemenid empire. Since most of the fighting was done on horseback, we would expect the commanders of the Companion cavalry to be prominent in our sources, but they are conspicuously absent. The reason is that the unit of Hephaestion and Clitus was too large for the guerilla warfare in Sogdia. It was divided into smaller units and Hephaestion commanded one of them. Clitus was appointed as future satrap of Bactria and Sogdia, an incredibly important function. (In the Achaemenid empire, this task was left to the crown prince.)
In the autumn of 328, however, a tragic incident took place. During a drinking party in the satrapial palace at Maracanda in Sogdia, many courtiers were flattering Alexander, who had won a difficult war in the Sogdianian desert. Some called him the son of Zeus Ammon and belittled Alexander's human father Philip, others made jokes about the commanders who had been defeated and killed by the native leader Spitamenes. This was more than Clitus, who had served under Philip and knew the dead commanders, could stomach: he started to praise Philip.
Hearing his words, Alexander felt offended, and in his drunken rage pushed aside his bodyguards Ptolemy and Perdiccas and run a lance through Clitus, who died on the spot.
When the king was sober again, he understood that he had made one of the greatest mistakes of his life. For three days, he considered suicide but then he decided to accept life again. The fact that the officers had constructed some evidence to suggest that Clitus was a traitor, may have helped. The same can be said about the court philosopher Anaxarchus of Abdera, who told him that the king was justice impersonated, and could therefore never act badly.
This may have been comfortable to Alexander, but set a new standard of flattery. From now on, hardly anybody dared to correct Alexander any more.
Clitus is sometimes called "Clitus the Black" to distinguish him from another Macedonian officer ("the White"), who rose to prominence after Alexander's death.