He availed himself of his influence as master of the palace, and as husband of Sophia, the niece of the late empress Theodora, to secure a peaceful election. The first few days of his reign--when he paid his uncle's debts, administered justice in person, and proclaimed universal religious toleration--gave bright promise, but in the face of the lawless aristocracy and defiant governors of provinces he effected few subsequent reforms.
The most important event of his reign was the invasion of Italy by the Lombards, who, entering in 568, under Alboin, in a few years made themselves masters of nearly the entire country. Justin's attention was distracted from Italy towards the North and East frontiers. After refusing to pay the Avars tribute, he fought several unsuccessful campaigns against them. In 572 his overtures to the Turks led to a war with Persia. After two disastrous campaigns, in which the Persians overran Syria, Justin bought a precarious peace by payment of a yearly tribute. The temporary fits of insanity into which he fell warned him to name a colleague. Passing over his own relatives, he raised, on the advice of Sophia, the general Tiberius to be Caesar in December 574 and withdrew into retirement. Sophia and Tiberius ruled together as joint regents for four years, while Justin sank into growing insanity. He died in 578.