Gustavus returned to Mora and was made the leader of the peasants in that locality. With these men he started his work of liberation, which was the commencement of one of the most remarkable of revolutions that the world ever saw. In the beginning of February, 1521, Gustavus marched southward with a few hundred men. At Falun he captured the bailiff of the mines, confiscating the royal taxes. Returning to the starting point, he left it again, with an army of 1,500 men. Entering Norrland, where he was joined by the peasants of Gestrikland, and the burghers of Gefle, while the people of Helsingland asked for time to consider the matter, he learned upon his return how one of his commanders, Peder Swensson, had won a glorious victory over a Danish army 6,000 strong at the ferry of Brunnbæck, by the Dal River. Gustavus began training his troops, enforcing severe discipline and providing them with better arrows and longer lances. He declared war upon Christian in a formal way and marched on Westeros, where the Danish troops had centred. The town and castle were captured in spite of a force of superior Danish cavalry.
Gustavus shifted his army into divisions which marched in various directions to capture the castles of surrounding provinces. The people of Upland reinforced the Dalecarlians, who were sent home to tend to their sowing. The Upland forces captured the archbishop’s seat during his absence, and were joined by Gustavus at Upsala, who made an exceedingly severe speech to the ecclesiastics, asking them to decide their nationality, whether they were Swedes or not. They asked permission to consult Archbishop Trolle, which was granted. “I will bring the reply myself,” said Trolle, starting from Stockholm with a splendid body of German troops. Gustavus was near being taken by surprise, but gathering troops he fought the archbishop, whose force met with a crushing defeat, and he escaped with difficulty to Stockholm.
At midsummer, 1521, Gustavus arrived at Brunkeberg, laying siege to Stockholm. The capital was strongly fortified, and Norrby with a Danish fleet supported and relieved it. Twice the Danes routed the Swedish troops with the intermission of one year, but Gustavus provided reinforcements. He travelled through the country, visiting the forces who laid siege to the various Danish strongholds, these surrendering one by one. It was not a chain of glorious exploits, this work which Gustavus carried to a successful end, but one of infinite patience and sagacity, saddened by the news that the revengeful Christian had ended the lives of his captive mother and sister in the miserable Danish dungeon. Bishop Brask was scared into submission, turning his castle Stegeborg and part of his troops over to Gustavus, who at a Riksdag at Vadstena was elected regent in August, 1521.