The Kalmar Union was both good and bad for Sweden depending on the time and the strength of their ruler. However, slowly they lost power to Denmark and in 1520 Denmark essentially invaded the country to assert that dominance. This led the Swedish people to rise up and crown a new king, Gustav I. King Gustav quickly altered the country, most notably by introducing Protestantism and by breaking up the Hanseatic League's monopoly over Swedish trade.
The 1600s brought Sweden to the forefront of Europe as the Swedes entered mainland Europe and took lands from the Russians, Poles-Lithuanians, and the Holy Roman States among others. Soon, Sweden was one of the largest countries in Europe, but the constant war (for nearly half a century) had depleted the treasury, although it also gave Sweden the best military in Europe.
In 1700 the wars began again, most notably the Great Northern War with Russia. These wars struck in the east and south, invading both Poland and Russia. However, the Swedes lost their battles with the Russians and only 10 years later had fallen from the ranks of Europe's powers. This was quickly proven when Sweden lost a battle against neighboring Norway.
This decline continued in the 1800s when Sweden lost their territory of Finland to the Russians. With the Napoleonic wars, Sweden fought the French and upon their defeat Sweden was granted the country of Norway, which was then solidified with a war victory over the Norwegians. From this point on, the century remained peaceful as the country focused on domestic affairs and growth. In the late 1800s the industrial revolution arrived in Sweden and it quickly expanded its cities and economy even further.