Second Swedish Crusade The Second Swedish Crusade was a Swedish military expedition to areas in present - day Finland by Birger Jarl in the 13th century . As a result of the crusade , Finland became part of Sweden for the next 550 years . Year of the crusade . According to " Eric ' s Chronicle " from ca . 1320 - 1340 , the crusade took place between Birger Jarl getting elevated to the position of jarl in 1248 and the death of King Eric XI of Sweden in 1250 . The so - called " Detmar Chronicle " of Lübeck from around 1340 confirms the expedition with a short note that Birger jarl submitted Finland under Swedish rule . From other sources , Birger jarl is known to have been absent from Sweden in winter 1249 - 50 . Later on , the conquest of Finland was redated to 1150s by the official Swedish legends , crediting the national saint King Eric for it . The point of time when the attack took place has been somewhat disputed . Attempts have been made to date the attack either to 1239 or to 1256 . Neither date has received wide acceptance . Background . Sweden ' s sudden determination to take over Finland has not been explained , but for one reason or another Finland was high on Birger Jarl ' s agenda . Finland became an integrated part of Sweden since there was a lot of exchange between the regions , especially via the Åland archipelago . During those days , it was easier to travel by sea than by land . Birger Jarl seems to have headed for Finland just after having both crushed the Folkung uprising of 1247 - 1248 and finalized the Treaty of Lödöse with Norway earlier in the summer of 1249 . Sweden ' s previous attempts to gain a foothold in Estonia in 1220 may have urged Sweden to settle with what was still available . Eric ' s Chronicle also points out the threat from Russians , mentioning that the " Russian king " had now lost the conquered land . Target of the crusade . All details of the crusade are from Eric ' s Chronicle , which is largely propagandist in nature , written amidst internal unrest and war against Novgorod . The chronicle has caused a long controversy on the actual target of the expedition , since it presents Tavastians ( taffwesta ) as the Swedish opponents . Based on this , it is usually assumed that the target of the crusade was also Tavastia , even though that is not explicitly said in the chronicle . Tavastians are known to have rebelled against the church in the 1230s , which had resulted in a papal demand for a crusade against them in a letter in 1237 . According to the chronicle , the expedition was prepared in Sweden and then conducted over sea to a land on the coast , where the enemy was waiting . Since Tavastia was inland , this contradiction was later explained so that there was a Tavastian port somewhere on the coast that was the primary target of the attack . Chronicle also mentions that a castle called " taffwesta borg " was established after the war . There have been lot of attempts to identify the castle with either Häme Castle or Hakoinen Castle in central Tavastia , but neither has been indisputably dated to such an early period . The first Swedish garrisons in Finland seem to have been in a hill fort later called as " Old Castle of Lieto " , not far from Turku and Koroinen , the fortified church - residence of the early bishops , along the Oxen Way to central Tavastia . Church reaction and reorganization . Probably related to preventing other parties from getting involved in the conflict , Pope Innocent IV took Finland under his special protection in August , 1249 , however without mentioning Sweden in any way . Finland ' s bishop Thomas , probably a Dominican monk , had resigned already in 1245 and died three years later in a Dominican convent in Gotland . The seat being vacant , the diocese had probably been under the direct command of the papal legate William of Modena whose last orders to Finnish priests were given in June , 1248 . Swedish Bero was eventually appointed as the new bishop in 1248 / 9 , presumably soon after William ' s visit to Sweden for an important church meeting at Skänninge that ended on March 1 , 1248 . The so - called " Palmsköld booklet " from 1448 noted that it was Bero who gave Finns ' tax to the Swedish king . Bero came directly from the Swedish court like his two successors . It seems that Swedish bishops also held all secular power in Finland until 1280s when the position of the Duke of Finland was established . In 1249 , the situation was also seen clear enough to have the first Dominican convent established in Finland . There had been no monasteries in Finland before that . The convent was situated next to the bishop ' s fortification in Koroinen until the end of the century . Aftermath . As an unexpected side effect , the expedition seems to have cost Birger the Swedish crown . As King Eric died in 1250 and Birger was still absent from Sweden , the rebellious Swedish lords selected Birger ' s under - aged son Valdemar as the new king instead of the powerful jarl himself . From 1249 onwards , sources generally regard Finland as a part of Sweden . Diocese of Finland is first listed among the Swedish dioceses in 1253 . In Russian chronicles , the first reliable mention of Finns being a part of Swedish forces is from 1256 . However , very little is known about the situation in Finland during the following decades . Reason for this is partly the fact that Finland was now ruled from Turku and most of the documentation remained there . As the Novgorod forces burned the city in 1318 during the Swedish - Novgorodian Wars , very little remained about what had happened in the previous century .