In classical times before Christianisation, Rome had an informal, private divorce process.  Divorces could be carried out mutually by the partners.  Husbands could unilaterally decide on divorce for little or no reason, announced by a letter "repudium". 

In 449 the emperors Theodosius and Valentinian of Rome changed the divorce law to allow penalty-free divorces to men and woman if their spouse committed certain acts (homicide, poisoning, robbery, etc). In addition, husbands were specifically allowed to divorce their wife, keep the dowry and remarry later if he could prove that she was: "(I) going to dine with men other than her relations without the knowledge or against the wish of her husband; (2) going from home at night against his wish without reasonable cause; (3) frequenting the circus, theatre or amphitheatre after being forbidden by her husband." 

It was only in the 700s that the Catholic Church announced that marriage was indissoluble by divorce or death. Annulments and dissolutions of marriage were conducted in a limited way until the 1100s when marriages were enforced strictly and even adultery could only result in separation, not divorce.  Annulments were possible if the parties could prove they were too closely related by blood, and since noble houses were often closely related it could be conveniently exercised as a way to divorce.  An online article about family law gives a fascinating excerpt from a knight's letter in the 1100s commenting on his wife-to-be: "Without any doubt she is related to me within the third degree. That is not close enough to stay away from her. But if I want, and if she does not suit me, I can, on the basis of this relationship, obtain a divorce." 

In 1974 the government of Italy legalized divorce.

Source: http://molly.kalafut.org/marriage/divorce.html