The Lombard League was formed in 1167, largely out of the Veronese League. It was a Union of Lombard cities promising each other unity, against Emperor Frederick I Barbarossa. The cities of Lombardy swore their oath of mutual protection at Pontida, a small village in the region.
After the disastrous defeat of Pope Alexander III at the Battle of Monte Porzio in May 1167 by the imperial forces, the Lombard League remained as the last legitimate fighting force opposing the emperor and was therefore heavily backed by the pope.
In September 1174, Frederick embarked on his fifth Italian campaign, to quash the constant revolts in Lombardy and settle his quarrels with Pope Alexander III. Frederick led a force of 8,000 knights over the Alps and arrived in Piedmont in late September. His cousin Henry the Lion and his forces were once again not a part of the imperial campaign. Frederick wanted to take revenge on Susa for its behaviour of 1168, and on September 30 his forces captured and burned down the town. His next goal was the town of Asti, which he captured after a seven-day siege. In October, Frederick finally received the promised imperial reinforcements from Bohemia. Upon Frederick's rapid and fierce initial success, Margrave William of Montferrat and the Count of Biandrate abandoned the Lombard League.
The siege of Alessandria was an important event in Frederick's fifth campaign as this was a campaign of revenge, with the aim of the total destruction of the Lombard League and the removal of the Pope Alexander III. Frederick's next goal was therefore the Lombard city of Alessandria. Alessandria was founded by Milanese refugees, who fled after Frederick's forces burned and destroyed the city of Milan in 1162 and named after Pope Alexander III. The siege of the "Straw City", called so because all the roofs were covered with straw, began at the end of October. To Frederick's surprise and anger, his forces were not able to take the city so he had to spend the winter in front of its gates. On Holy Saturday, Frederick's forces managed to enter the city by digging tunnels under its walls, but the attack was repulsed by the Milanese with heavy losses. Alessandria withstood, and that was the first victory of the Lombard League. Frederick had to break off the siege due to an advancing Lombard army and retreated to Pavia.