The investiture controversy, the most significant conflict between Church and state in medieval Europe, begins as a power struggle between Pope Gregory VII and Henry IV.

An early result is the trek of Henry from Speyer to the fortress at Canossa in Emilia Romagna to obtain the revocation of the excommunication imposed on him by the Pope.

The Emperor is in January 1077 forced to humiliate himself on his knees waiting for three days and three nights, before the entrance gate of the castle, while a blizzard rages.

Henry enters Pavia and is crowned as King of Italy, receiving the Iron Crown of Lombardy.

He assigns a series of privileges to the Italian cities who have supported him, marches against the hated Matilda of Tuscany, declaring her deposed for lese majesty, and confiscates her possessions.

Henry next moves to Rome, which he besieges first in 1081: he is compelled to retire to …

…Tuscany, however, where he grants privileges to various cities and obtains monetary assistance (three hundred and sixty thousand gold pieces) from a new ally, the eastern emperor, Alexios I Komnenos, who aims to thwart Norman efforts against his empire.

Kastoríá surrenders, despite the fact that its English garrison consists of three hundred fiercely anti-Norman members of Emperor Alexios' Varangian Guard.

Pope Gregory VII, however, suddenly recalls Guiscard to Italy to help him expel the German king Henry, who seeks a military decision in his struggle with the church.

Leaving his forces under the command of his son Bohémond, Guiscard sails for Italy.

A second and equally unsuccessful attack on Rome is followed by a war of devastation in northern Italy with the adherents of Matilda.

Towards the end of 1082, the king makes a third attack on Rome.

Bohémond continues the Norman advance, ravaging parts of Greece, including Thebes and ...

…Corinth.

Alex uses Melissenos as the centerpiece to a ruse at Larissa, in Thessaly.

The emperor gives him the imperial insignia and a detachment of the army, which Bohemond proceeds to attack in the belief that this is the main imperial force, since the emperor is present with it.

While the Normans pursue Melissenos's men, Alexios with the main army takes and loots the Norman camp, forcing Bohemond to lift the siege and withdraw.

Many of his dispirited, homesick men, demoralized by the huge rewards that Alexios offers deserters, fall away.

Bohemond goes to Italy to raise more money but his principal lieutenants surrender almost as soon as he has departed.

A Venetian fleet has recaptured Durazzo and …