The Northern Wars (1655-1661) is a name sometimes used for the series of conflicts between Sweden and its adversaries Polish-Lithuanian Commonwealth (The Deluge, 1655-1660), Russia (1656-1661), Brandenburg-Prussia (1657-1660), the Holy Roman Empire (1657-60) and Denmark (1657-1658, 1658-1660). The same wars are sometimes referred differently in other countries: for example in Poland The Deluge sometimes is a name for the series of war against Sweden, Brandenburg, Russia, Siebenbürgen and Cossacks.
Therefore, the Northernwars are of great importance for anybody interested in the processes of European state formation, the development of the European state system, or those changes in the ways of war which are often labelled 'The Military Revolution'.
The Swedish-Russian and Swedish-Prussian wars during the eighteenth century were indeed very different from the wars of the previous centuries.
It works fine to treat the Danish-Swedish wars as a kind of side-show  even if this side-show had its own roots in the breakdown of the medieval Nordic union, and the fighting was on a scale equal to that on the main theatre.