Early Finnish wars Scattered descriptions on wars against Finland or between the Finns and the neighboring peoples prior to Finland becoming a part of Sweden has survived in Icelandic sagas , German , Norwegian , Danish and Russian chronicle s and Swedish legends . Most of the early Finnish wars remain unhistorical and cannot be verified from reliable sources . Conflicts with Swedes . Saga sources . The Ynglinga saga tells about the first known military expedition to Finland . Based on the saga ' s internal chronologies , the war presumably took place at the end of the 4th century , six generations before the time of the semi - historical Swedish king Ohthere ( Vendelkråka ) . However , it is disputed whether the Old Norse concept of " Finland " is always meant to refer to the approximate area of the present - day country of Finland ; alternatively it could have in some instances meant the land of the Sámi , or Finnmark ( " mark " meaning land ) in Northern Scandinavia . The Norna - Gests þáttr saga tells that Finnic / Finnish Kvens and the Curonians were raiding in Sweden in the mid - 8th century . In 862 , some Finnic and Slavic tribes rebelled against the Varangian Rus ' , driving them overseas back to Scandinavia . This took place according to the Primary Chronicle . King Eric Anundsson is told to have conquered Finland in the late 9th century , along with several other eastern countries . However , all other accounts of the king exclude Finland from his conquests : In 1187 , the royal and commercial centre of the Sveas ( Swedes ) Sigtuna was raided and burned by attack from easterly direction , according to Eric ' s Chronicle in 1335 . The Karelians are blamed for the offense . In 1216 , the Danish historian Saxo Grammaticus writes in Gesta Danorum about kings who ruled over Finland and Kvenland and about the Scandinavian royal families which based on several medieval sources descend from them . Grammaticus ' writings share a likeness and many characters and stories with the writings of Snorri Sturluson . Based on Grammaticus , many heroic Scandinavian figures have Finnic roots . Of the legendary Battle of Bråvalla ( c . 750 ) - Sveas ( Swedes ) against the Geats - he writes : German sources . Terra Feminarum , described in " Gesta Hammaburgensis Ecclesiae Pontificum " ( Deeds of Bishops of the Hamburg Church ) by Adam of Bremen in 1075 , is presumed to refer to the Finnic Kvenland , which at the time covered a part of the modern - day Finland . Terra Feminarum was attacked by the " " Sueones " " ( Swedes ) in 1050s . According to the source , the attack ended in a Swedish defeat and led to a death of the king ' s son who was in charge of the campaign . The details given on the conflict are however convoluted . Medieval Swedish legends . An alleged Swedish war against Finland took place in the 1150s , known as the First Swedish Crusade . Whether it ever actually happened is however disputed , as the information is based on a late 13th century legend , which has been claimed untrustworthy . The southern part of the modern - day Finland became an integrated part of Sweden from early on . According to Eric ' s Chronicle from the 1320s , that happened as a result of a Swedish crusade to the southern part of the modern - day Finland in c . 1250 . However , no other document discusses the crusade , and Eric ' s Chronicle provides contradicting information . The account is largely propagandist in nature , written amidst internal unrest and war against Novgorod . Conflicts with Norwegians . In 873 , according to the Egil ' s saga - written in c . 1240 - , the Kvens and Norse cooperated in battling against the invading Karelians . The chapter XVII of Egil ' s saga describes how Thorolf Kveldulfsson ( King of Norway ' s tax chief starting 872 AD ) from Namdalen , located in the southernmost tip of the historic Hålogaland , goes to Kvenland again . Based on medieval documents , the meeting took place during the winter of 873 - 874 : In 885 ( c . ) , the " Account of the Viking Othere " tells that the Norwegian and the Kvens were in conflict with each other from time to time : The account is a report based on Ohthere ' s ( Ottar ) voyage of the oceanic coasts of Northern Scandinavia and today ' s extreme northwestern Russia . The Kvens are referred to as " Cwenas " who live in " Cwena land " . This was the first genuine account about the North . Thus , it is a principal source in studies relating to the Nordic history . In 890 , Ottar reports the findings to King Alfred of Wessex , who has Ottar ' s account included to the omissions and additions added to the " Universal History of Orosius " , republished by King Alfred . The book is partially a work of Orosius and partially of King Alfred . The " Kven Sea " is mentioned as the northern border for ancient Germany . The location of Kvenland is also explained in the following ways : In 1007 ( c ) , King Olaf II of Norway ( Olaf II Haraldsson , " Saint Olaf " , 995 – July 29 , 1030 ) plundered in Finland and almost got himself killed at the Battle at Herdaler , according to the " Saga of Olaf Haraldson " , a saga within the Heimskringla saga . In 1100s - according to Egil ' s Saga from c . 1240 - , the Norwegians and the Kvens cooperate against the expansionism of the Karelians and in the taxation of the Samis . In 1230 , the Orkneyinga saga tells that the Southern Norwegian lake district , including Lake Mjøsa ( 100 kilometers north from the modern - day Oslo ) , were attacked by men from Kvenland . In 1251 , the Karelians fight against the Norwegians . In 1271 , the Kvens and the Karelians cooperate in battles against the Norwegians in Hålogaland , in Northern Norway , according to Icelandic chronicles . These battles had a lasting effect in the geopolitical landscape of the entire Northern Scandinavia : Conflicts with Danes . Vague chronicle entries shortly mention Danish expeditions to Finland in the 1190s and in 1202 . Nothing is known about their results except what can be read from a papal letter from 1209 to the Archbishop of Lund , which lets the reader understand the church in Finland to be at least partly established by Danish efforts . Conflicts with Novgorod . The mid - 12th century was a very violent time in the northern Baltic Sea , with the Finns and Swedes in frequent conflicts with Novgorod ; The series of wars waged during the 12th and early 13th centuries between Finnic groups and the Republic of Novgorod probably contributed to the Finns ' eventual subjugation to the Catholic Church and the Kingdom of Sweden .