The Swedish Great Power period in the 17th century was made possible because of both internal and external causes. In the middle of the 16th century, when the German Order State ceased to exist, Sweden, Denmark, Russia and Poland competed for influence over territories in the Baltic area; territories that earlier belonged to the German Order. Sweden managed to conquer the northern part of Estonia and this was the beginning of a very expansive Swedish foreign policy. The expansion was made easier by the Civil War that raged in Russia in the early years of the 17th century. Sweden took part in this war, trying to take advantage of the chaotic situation. The peace treaty added new territory to the Finnish part of Sweden.
The reason why the states surrounding the Baltic Sea were so eager to take control of the Baltic states, was their importance in being able to control the trade from Russia to western Europe. The collapse of the German Order State created a political vacuum that constituted a threat to trade relations. Because of the political turbulence in the Baltic area, the English established a new trade route north of Scandinavia, to Russia. When Sweden, because of this, tried to take control of the northern coastal regions of Scandinavia, it led to conflict with Russia and Denmark. When King Gustav II Adolf, who "inherited" the conflict, realised that it would be very difficult for Sweden to control the northern route, he concentrated the Swedish interest to the Baltic Sea. Sweden now strove to create a "dominium maris baltici", i.e. supremacy of the Baltic Sea.
The first problem was the trade from Russia to western Europe. Most of it went from Russia over "Livland", i.e. modern Latvia, to cities in western Europe. In the early 17th century "Livland", and the coastal areas in the south-west of the Baltic Sea, were controlled by Poland. This meant important customs revenues from the commerce relations with Russia. The Swedish king was, of course, interested in getting his hands on these revenues. The supremacy over "Livland" and other important harbour cities in polish Prussia, meant massively increased revenues for the Swedish state. The customs revenues alone, amounted to over 25 percent of the Swedish state income.
Internal factors that had an impact on the development towards a great power were, among other things, a fairly well managed economy, a growing domestic iron and weapons industry, a, for the time, very well-administrated state and a slightly increasing population. However, the external factors, a weak Russia and social and economic problems in other European countries that competed with Sweden, are regarded by most scholars as the main explanation as to why Sweden, with its limited resources, could become so powerful.