Gradually a process of immigration started bringing to the Mezzogiorno Norman knights and warriors, and more generally, ‘Franks’ (from different regions of the kingdom of France), and also Italians from the north, in the pay of different local lords. The Normans became increasingly important politically and militarily in the Mediterranean region. In this period and in this region, it was usual that they became mercenaries. The attack on Apulia began in May 1017 with the invasion of the Capitanata, the area where Byzantine rule was least firmly established. The initial engagements were won by Melus and his supporters but, in contrast to the earlier rebellion in 1009, the Byzantines kept control of the coastal towns. Both sides sought reinforcements and both Glaber and Amatus imply that a second and larger group of Normans joined the original forces.
The delay worked to the advantage of the Byzantines and the new catepan, Basil Boiannes, who arrived at Bari in December 1017 with substantial forces. The timing of the rebellion was unfortunate as the Emperor Basil II has succeeded in his conquest of Bulgaria and with the frontiers at peace, Byzantium had troops to spare. Boiannes’ approach to the problem took two forms. First, he undermined the support Melus was getting from the Lombard princes: for example, he confirmed the property rights of the abbey of Montecassino in Apulia, ruled by Atenulf, the younger brother of the prince of Capua. Secondly, he pursued a vigorous military campaign defeating Melus at Canne, a few miles north-west of Trano in October 1018.
According to Leo Marsicanus only eighty of perhaps three hundred Normans survived the battle. They took service with the Lombard princes, the abbot of Montecassino or even with the Byzantines. Melus went north to Germany to enlist the help of the Emperor Henry II, where he died in April 1020. Boiannes consolidated his victory by constructing a series of fortified settlements across northern Apulia: Troia, Fiorento, Montecorvino, Dragonara and Civitate. Further south he built a new town at Melfi on the border of Apulia and the principality of Salerno. He also improved Byzantine relations with the Lombard princes: Guaimar III of Salerno was always reasonably disposed towards the eastern empire and Pandulf IV of Capua was easily bribed. Byzantine success led to intervention by the German emperor in 1022, conscious of his predecessor’s claims of overlordship of southern Italy. Pandulf IV of Capua was deposed and narrowly escaped execution; his brother Atenulf, the abbot of Montecassino drowned in the Adriatic as he fled to Constantinople. Pandulf was replaced by his cousin count Pandulf of Teano and a pro-imperial abbot, Theobald was installed at Montecassino. German military action in Apulia in the summer of 1022 stalled before the new fortress of Troina and Henry soon began his withdrawal north. Henry II died in 1024 and his successor Conrad II released Pandulf IV who, within a few years had regained control of Capua. The abbey of Montecassino was now exposed to Pandulf’s revenge.
The remainder of the 1020s and the 1030s were dominated by two things. There were bitter disputes between and within the different principalities and duches of the west and centre of the peninsula though the principality of Salerno remained considerably more stable than its neighbours. Certain themes can be identified: the struggle of Pandulf IV to reassert his control over Capua, the attempts by the Lombard principalities to absorb the independent coastal duchies and the comtinued ambition of the princes, especially those of Salerno and Capua to assert their dominance within the Lombard territories, something Guaimar IV of Salerno achieved in 1038. In this period, Norman mercenaries served the Lombard princes in the principalities of Capua, Naples, Salerno and Benevento reaping considerable benefits as a result. This period also saw the enhanced power and prestige of the Byzantine Empire in southern Italy. With the Apulian border secure, Byzantine influence stretched into the Lombard areas. Basil Boiannes provided troops to assist Pandulf IV’s siege of Capua in 1025-6. There was a serious attempt to defuse relations with Rome. In 1025, the jurisdiction of the papacy over the ecclesiastical province of Bari was conceded followed five years later by the new bishopric of Troia. The Byzantine governors maintained strict control over the Latin churchmen in areas under their rule. This was essential as there were still some tensions in Apulia, where a Lombard population was under Greek rule.
Pandulf IV proved to be the most disruptive force in southern Italy. His ambitions were initially helped by the succession of his nephew Guaimar IV to the principality of Salerno in March 1027. Guaimar did not interfere when Pandulf seized Naples in 1027, though the Capuans were driven out of the city within three years. This did not prove the end of his ambitions. In 1036, he tried to wrest Benevento from his other nephew Pandulf III and soon after he briefly took over the duchy of Gaeta. However, Pandulf IV faced problems within his principality of Capua. To maintain their authority in the north of the principality, the traditional princely approach was an alliance with, or control over, the wealthy and powerful monasteries of Montecassino and St Vincent on Volturno. Abbot Theobald as the nominee of the emperor was not considered reliable and, though he remained abbot, he was forced to live, effectively as a prisoner at the Cassinese cell in Capua. Though he escaped into the Abruzzi outside the prince’s control and did not die until 1035, effective power with Pandulf’s supporters, a lay official called Theodwin and with Basil, provost of St Benedict, Capua who became abbot in 1035. There were similar complaints about the prince’s oppression of the monastery of St Vincent.
The 1020s brought confirmation of the Norman presence in Campania. Some of the survivors of the debacle of 1018 were taken into service by the prince of Benevento. Abbot Atenulf of Montecassino stationed a garrison of Normans in a fortress at Pignetaro to oppose the counts of Aquino and the troops given by Guaimar III to his brother-in-law Pandulf in 1025 were Norman. The most important development was associated with the Normans led by Rainulf, possible one of the brothers of Gilbert Buatère who had come to Italy in the 1010s. Having served various lords for ten years, for which there is little evidence Rainulf Drengot was enlisted by the duke of Naples, Sergius IV, himself forced to flee by the Lombard prince of Capua, Pandulf IV in 1027. Back on his throne in 1030, the duke of Naples granted the title of ‘count’ to Rainulf, the town of Aversa and its lands, the important strategic centre of Liburia, and the rich Terra del Lavarro (a vast plain between Naples and Caserta). Rainulf Drengot also married the duke of Naples’ daughter. Thus, the Norman, by a complex game of opportunity and alliances succeeded in a few years to reinforce his position and establish his county in Aversa. This settlement was the first real base that the Normans possessed in southern Italy that was under their control rather than by a local ruler.
After taking control of Aversa in 1030, Rainulf Drengot accelerated the process of immigration of new compatriots, a necessary step to maintain order and govern a population judged unreliable. He integrated easily into the local political order of southern Italy yet did not hesitate to change sides if that served his interests. Thus, when his wife died in 1034, he rebuffed his brother-in-law and protector, the duke of Naples, by marrying another Lombard princess, Pandulf IV of Capua’s niece, the daughter of the duke of Amalfi. From this alliance, Rainulf of Aversa gained territory taken from the possessions of the abbey of Montecassino. Soon after, in 1039, he was on the side of Guaimar IV of Salerno, supported by the troops of the German Empire led by Emperor Conrad II. Rainulf of Aversa defeated Pandulf of Capua, annexed his land, and with the approval of Conrad II, reunited the two principalities under one crown, thereby becoming the ruler of the largest political entity in Mezzogiorno. Thereafter, the county of Aversa, the first long-lasting Norman principality in southern Italy, rapidly became a real force that could only be rivalled by that other great Norman dynasty, the Hautevilles.
In 1038, the situation in southern Italy was transformed by two events: the expedition to southern Italy by the emperor Conrad II and renewed attempts by the Byzantines to reconqueror Sicily. Both were to have profound consequences. Conrad II wished to reassert his control over the Lombard principalities but his expedition appears to have been in response to complaints about the activities of Pandulf IV. Pandulf sought to limit the damage by offering Conrad hostages and paying a substantial tribute and it was not until he failed to fulfil his promises that Conrad acted against him. In May 1038, he invested Guaimar IV of Salerno with the principality of Capua. Pandulf sought aid from the Byzantine court at Constantinople but was disappointed and remained in exile for two years. Guaimar rapidly extended his authority over the neighbouring city states: he took over Amalfi in April 1039 and Sorrento four months later. In 1040, he briefly controlled Gaeta. Count Rainulf certainly aided Guaimar in his annexation of Sorrento but the latter’s success cannot be entirely put down to Norman military aid: there were internal disputes in the ruling families in both Amalfi and Sorrento. Guaimar’s control over the Normans appears to have been limited. When Richer, the new imperial-appointed abbot of Montecassino complained to Guaimar about Norman attacks on the abbey’s lands, he was advised to seek aid from the emperor.
In the late summer of 1038, the Byzantine emperor organised a great expedition against the Saracens of Sicily, who were divided and weakened by dynastic quarrels. It was led by George Maniakes who appealed to the Italians, who were on good terms with Emperor Michael IV the Paphlagonian (1034-1041). Guaimar IV of Salerno sent, under the command of a north Italian from Milan, Arduin a contingent of three hundred Normans including William Iron-Arm and his brother Drogo de Hauteville. Its task was to reinforce the imperial army, and also to get rid of mercenaries, always dangerous and a nuisance during periods of peace. In the battle, the Normans of Italy encountered Scandinavian warriors from the Byzantine imperial guard, the Varangian guard led by Harold Hardrada, who would become king of Norway in 1047 and rival of Harold of England and William of Normandy in 1066. Other Norman adventurers such as Roussel de Bailleul were part of the expeditionary corps around Reggio in Calabria before the embarkation. The expedition lasted more than two years. Messina was taken in autumn 1038; victorious campaigns followed one after the other in the north, west and centre of the island. The audacious reputation of the Normans was maintained. But they remained simple mercenaries. The relations with their superiors deteriorated after the recall of George Maniakes and unhappy with their pay and spoils, the Normans and the Varangians abandoned the Greek army in 1040. The Byzantines were forced to retreat and soon only Messina was left under their control. Even though they were simple players in the hands of a foreign power, this occasion represented the first recognition for the Normans and would have important consequences for Sicily.
These years represented an important stage in the adaptation of the Norman mercenaries to political conditions in southern Italy. They were able to assess the military capabilities of Lombard princes and the Byzantine Empire as well as making substantial profits from their fighting abilities. Norman success in southern Italy was not simply a military success; it also represented significant political success.
 William de Hauteville was born in Hauteville-la-Guichard, Normandy and died in the winter of 1045-1046. He was a Norman adventurer, the eldest of twelve Hauteville brothers, a soldier of fortune who led the first contingent of his family from Normandy to southern Italy. William and his brothers Drogo and Humphrey responded (c. 1035) to an appeal for reinforcements in Italy by the Norman Rainulf of Aversa. William earned his nickname “Iron Arm” during the Norman-Byzantine siege of Muslim-occupied Syracuse in Sicily when he charged and killed the emir of the city. He served as a captain of the Norman army that joined the Lombards in invading Apulia, in southern Italy, and was proclaimed count of Apulia in 1042. Guaimar IV, the Lombard prince of Salerno, confirmed the title later that year. Guaimar arranged a marriage between William and his own niece, daughter of the Duke of Sorrento. Emerging as the most powerful leader in southern Italy, William allied with Guaimar, invaded Calabria two years later. After his death, his brother Drogo became count of Apulia.
 Exiled for some reason, Roussel de Bailleul is an example of the Norman adventurers who left to seek fortune in the Mediterranean. He fought for Roger de Hauteville in 1063 in Sicily at the battle of Cerami. He then left to join the troops of mercenaries to the service of the Byzantine Empire. Benefiting from the great defeat against the Turks with Manzikert in 1071, Roussel fought on his own account against the Turks and the Greeks. In 1072, Emperor Michael VII appealed to Sultan Sulaiman ibn Qutlumish for help against the Norman mercenary Roussel de Bailleul, who had rebelled and set up an independent principality in Byzantine territory. Roussel was defeated outside the walls of Constantinople by the combined armies, but the Turks had now achieved a foothold in Asia Minor. Captured by the Turks in 1074, Roussel paid his ransom and takes again its autonomous policy and continues his career as a mercenary until his mysterious death in 1078.