Welcome in the group dedicated to Sweden & Poland - not only in Hetalia terms! You'll be able to find here both Hetalia fanworks and arts connected with these countries in real life + the arts done by artists from these countries unrelated to these topics.
Remember that this pairing is not a crack - both in historical Hetalia terms and in Hima's canon!
In Hetalia canon these two met in 1620s, when Sweden decided to take Estonia & Latvia for himself. Poland didn't like it, which led to strange Polish-Swedish Wars, from which - to Poland's disappointment - Sweden quickly backtracked. As a result of the wars, Sweden took Estonia & a half of Latvia for himself.
Relations between Sweden & Poland in real life are more richer. They formed a personal union in years 1592-1599 (their common ruler was Sigismund III Vasa), but it ended badly (due to Sigismund's fanatic Catholicism, while Sweden was a fanatic protestant country). That - and the conflicts over Inflanty (nowadays Estonia & Latvia) - caused many Polish-Swedish wars in 17th century, from which the most significant is Deluge (1655-1660). During it Sweden invaded Poland and completely plundered it. In the end Poland defended its independence and chase Swedes away, but after this event Poland was never as strong as it was before. The times of Deluge and Sweden itself are mentioned in Polish national anthem, "Poland Is Not Yet Lost". Sweden caused also big Poland's devastation during Great Northern War, what weakened Poland even much, so it become a tibbit for its neighbors. We can say so, that Sweden's actions indirectly caused partitions of Poland
Much later, during World War II, Swedes accepted many refugees from Poland and gave them a shelter. Swedish and Polish agents were also often cooperating with each other during this war. Nowadays the relations between these countries are normal. From time to time Swedes give back works of art plundered during Deluge. Besides that, a Swedish power metal band, Sabaton, became very popular in Poland, after their songs about Polish heroic acts during World War II - "40:1" and "Uprising".