PART 1 - BEFORE THE CRUSADES

To understand papal attitudes to warfare, and the 6th commandment (which I assume is what you mean) we have to go back to the later days of the Western Roman Empire. From 303 to 313 the Roman emperors heavily persecuted Christians within the empire. They failed to halt its rapid expansion, but had caused many Christians to publicly renounce their faith in fear of their lives. In 311 the persecution largely stopped, and with the Edict of Milan in 313 the peoples of the empire were granted freedom of religion. However, those Christians who had failed to uphold their faith needed to be dealt with.

Most church figures were happy to readmit them, but not all. In particular, a large group of Christians in North Africa did not see sacraments given by lapsed Christians as valid. In Carthage, a bishop called Caecilian had been appointed by Felix of Aptungi in 311. Both men were believed to have handed over religious scriptures to the Romans during the persecution and assisted it, which in the eyes of many North African Christians rendered this appointment invalid. They elected their own bishop, Majorinus, who died in 313. He was replaced by Donatus, a charismatic clergyman who led the sect for 40 years, and gave them their name - the Donatists. From 311 onwards, there were two bishops of Carthage. Augstine of Hippo was particularly keen to persecute the Donatists, and constructed a theological framework to do so which became known as Just War.

The theology had certain caveats. War could not be fought pre-emptively, and it had to be strictly defensive. When he saw what the Roman Empire was doing to the Donatists - at his request - he actually thought it went too far. However, Augustine did not go into great detail regarding what criteria to judge a war by, and nobody would until Thomas Aquinas in the 13th century. Generally speaking, Just War before Aquinas was described in just one sentence, used in treatises on canon law:

“That war is just which is waged upon command in order to recover property or to repel attack”

That phrase, or something very close to it, was used by writers such as Bernard of Clairvaux (lead preacher of the Second Crusade) and Gratian (leading canon lawyer of the 12th century). That sentence could be interpreted quite narrowly, as Bernard did in his early career - he even argued that self-defence was theologically dubious - but it could also be interpreted very broadly. It's worth noting that the medieval church was rather uninterested in the 10 commandments. They were from the Old Testament, and in the eyes of most medieval canon lawyers, that made it inferior to New Testament material. Penitential literature of the Middle Ages rarely mention the 10 commandments. The Gospel of John was often used to justify military action, as you will see below, which provided a loophole to ignore it.

The narrow interpretation was the prevailing one until the 9th century. In the 9th century, Rome was sacked twice by Islamic pirates, and Vikings were causing havoc all over Europe. They targeted Christian institutions in particular, because they tended to have large amounts of gold and silver in their possession. Monasteries were a very popular target, since not only were they relatively rich, they were also unguarded. The papacy had to respond somehow, and sanctioned the creation of military forces to counter piracy. Pope Leo IV was particularly important, as he formed an alliance of Italian cities to fight pirates, culminating in a decisive victory for the Italians at the Battle of Ostia in 849. This move toward militancy was sustained as pirate raids gave way to the arrival of the Muslims in southern Italy in the 10th century, and then renewed raiding later in the century. Pope John X was particularly militant, and built a coalition of Italians and Byzantines to fight off a large band of Islamic raiders who had established a base in Italy. In the 11th century, the Normans took their place as the expansionist power of southern Italy. The Normans were even more concerning because they were very successful. Where the Muslims just raided, the Normans conquered. With the arrival of the Normans, the papacy began to become more involved in military affairs.

By the 11th century, the practise of the papacy calling upon secular rulers to form an army against a particular target was well established, and operated within a liberal framework of early Just War theology. Pope Leo IX personally led an army of Italians and Germans in an attempt to drive the Normans out of Italy at the request of the Byzantine Empire in 1053. Leo IX had not been personally under attack from the Normans, which marked a major shift in papal policy as this attack on the Normans was essentially a pre-emptive strike, albeit at someone else's request. This set the precedent that defence or liberation of others, at their invitation, was within the boundaries of the Just War. Leo's army suffered a catastrophic defeat at the Battle of Civitate on 15 June 1053. This campaign was a precursor to the First Crusade, which was also a request from the Byzantine Empire to drive out an expansionist power. This also cemented the idea that claims to land were as good as fighting to recover it. In 1066, William the Conqueror was given papal approval to invade England, as he had a claim to the kingdom.

In 1071, after being defeated by the Seljuk Turks at the Battle of Manzikert, the Byzantine Empire again reached out to the papacy in search of an army. In 1074, Gregory VII sent this letter to the Christian faithful, which shows how these military activities were pitched:

Gregory, bishop, servant of the servants of God, to all who are willing to defend the Christian faith, greeting and apostolic benediction.

We hereby inform you that the bearer of this letter, on his recent return from across the sea [from Palestine], came to Rome to visit us. He repeated what we had heard from many others, that a pagan race had overcome the Christians and with horrible cruelty had devastated everything almost to the walls of Constantinople, and were now governing the conquered lands with tyrannical violence, and that they had slain many thousands of Christians as if they were but sheep. If we love God and wish to be recognized as Christians, we should be filled with grief at the misfortune of this great empire and the murder of so many Christians. But simply to grieve is not our whole duty. The example of our Redeemer and the bond of fraternal love demand that we should lay down our lives to liberate them. "Because he has laid down his life for us: and we ought to lay down our lives for the brethren," [1 John 3:16]. Know, therefore, that we are trusting in the mercy of God and in the power of his might and that we are striving in all possible ways and making preparations to render aid to the [Byzantine Empire] as quickly as possible. Therefore we beseech you by the faith in which you are united through Christ in the adoption of the sons of God, and by the authority of St. Peter, prince of apostles, we admonish you that you be moved to proper compassion by the wounds and blood of your brethren and the danger of the aforesaid empire and that, for the sake of Christ, you undertake the difficult task of bearing aid to your brethren [the Greeks]. Send messengers to us at once and inform us of what God may inspire you to do in this matter.

So when the Byzantine Empire asked once again for aid against the Turks in 1095, Urban II acted within tradition. He then added the additional ingredient of remission of sins, where participants would have their sins washed away provided they acted well on the expedition. This was an explosive change, and one which required the papacy to confront the murder, rape, and pillaging conducted by soldiers acting in their name.