Battle of Monte Porzio

The Battle of Monte Porzio (also called the Battle of Tusculum) was fought on 29 May 1167 between the Holy Roman Empire and the Commune of Rome. The communal Roman army, which one historian has called the "greatest army which Rome had sent into the field in centuries", was defeated by the forces of the Emperor Frederick Barbarossa and his local allies, the Counts of Tusculum and the ruler of Albano. Comparing its effect on the city of Rome, one historian has been called Monte Porzio the "Cannae of the Middle Ages".

The site of the battle was the field between a small hill and the walls of the city of Tusculum, at a place called "Prataporci", about 25 km southeast of Rome. In his universal chronicle, the Chronica Universalis, the contemporary writer Sicard of Cremona describes the site of battle as "near Monte Porzio" (apud Montem Portium).

The Battle of Monte Porzio is part of the long struggle between the Italian city-states and the Holy Roman Empire. In 1166, Barbarossa set out on an expedition to Italy with the intent of deposing the anti-imperialist Pope Alexander III and setting up his own antipope, Paschal III. He also sent two eminent prelates of the Empire, Archbishop Rainald of Cologne and Archbishop Christian of Mainz, commanding armies into Latium (the region around Rome) to subdue those city-states still opposing the power of the emperor. On 18 May, Rainald took Civitavecchia and then moved into the friendly city of Tusculum, possibly at the suggestion of Count Raino, an imperialist. The communal Roman army had been harassing Tusculum, a longtime rival. Pope Alexander, knowing that Barbarossa was likely to come to Raino's assistance, urged the Romans to abstain from attacking his city. It did not work: when the consul (leader) of the Roman commune learned of the arrival of Rainald at Tusculum, he sent an army to besiege the archbishop in the city.

With the Roman army approaching, Count Raino and Archbishop Rainald sent word to Christian, who was away besieging Ancona on the coast, to come to their relief. Within Christian's army were the forces commanded by Bishop Alexander II of Liège, and Count Robert III of Loritello. The total number of troops Christian was leading was about 1,300, which, according to Otto of Sankt Blasien, was a combination of 500 knights (milites in contemporary Latin) and 800 caesarianos (imperial troops). Otto places 300 men inside Tusculum. Other chroniclers claimed the Christian had with him 1,000 cavalry and some Brabantine mercenaries. The lowest estimate of Christian's forces put it at 500 men.

Christian encamped his army beside the hill and rested for a day while trying to negotiate a resolution. The communal Roman army refused Christian's diplomatic overtures and instead attacked with their whole force, numbering 10,000 poorly-armed men, on Whitsunday. The name of the leader of the Roman force has not been preserved, but it may have been Oddo Frangipani. The imperial forces were gravely outnumbered, but they were more disciplined and better armed. The Brabantines were quickly routed, but the cavalry from Rainald's city, Cologne, withstood the charge of the Roman infantry. Two sallies from Tusculum divided the Romans: one hitting their flank and one running through the centre. As the Roman cavalry fled the field, the Brabantines descended on the Roman camp. Only a third of the Roman army had made it inside Rome's walls before nightfall. Thousands were eventually taken prisoner and sent to Viterbo (including a son of Oddo Frangipani), and more were left dead on the field and the road.

The pope and Oddo took refuge in the Colosseum (which at the time was fortified like a castle) and called in reinforcements. The city prepared for a siege. Later the pope fled to the city of Benevento and the Emperor entered Rome. The imperial army, however, was hard hit by a wave of either malaria or plague, and Barbarossa withdrew his forces to Germany.