Gustaf Vasa followed developments in the south with alarm. His break with Lübeck began to seem a serious miscalculation. The mutual defense pact negotiated with the Danish oligarchy months earlier had not been ratified. That spring Sweden had seemed to be Lübeck's primary target; Wullenweber had boasted that he would visit Sweden with a show that would not lack force. Old enemies in exile, Bernhard von Mehlen and Gustaf Trolle, signed on with Lübeck. The regime in Lübeck laid hands on Svante Sture and tried to persuade young Sture to claim the throne in Sweden. Gustaf Vasa protested that Lübeck was using young Sture to deceive common folk. The king's outbursts of abuse, rage, and violence increased alarmingly. Reluctantly His Grace instructed an envoy to ask the king's father-in-law, Duke Magnus of Sachsen-Lauenburg, to intercede with Lübeck “that the matter might be taken up in the spring so that We have time to arm Ourselves.” His Grace did not mean to back down or to offer concessions, but simply to buy time.

When the king got word of Lübeck's invasion of Holstein and Denmark, he fortified the coastal fortresses, prepared for attacks on Stockholm and Finland, and began recruiting in Livonia, Preussen, and Pomerania. He dispatched Swedish ships along the Baltic coast to protect Swedish trade, and to close the ports to Lübeck. He asked the Danish commander on Gotland to join the Swedish fleet and forestall Lübeck's attempts to snap up western traders off Reval.