Sweden's ambitious foreign policy brings it into conflict beginning in the mid-sixteenth century with the three other main powers that have an interest in the Baltic: Denmark, Poland, and Russia.

These three powers fight numerous wars with Sweden, which is at war for more than eighty of the last three hundred years it ruled Finland.

Finland itself is often the scene of military campaigns that are generally conducted as total war and thus include the devastation of the countryside and the killing of civilians.

One example of such campaigns is the war between Sweden and Russia that lasts from 1570 to 1595 and is known in Finland as the Long Wrath, because of the devastations inflicted on the country.

Sweden is also heavily involved in the Thirty Years' War (1618-48), in which the Swedes under King Gustavus II Adolphus thwarts the advance of the Habsburg Empire to the shores of the Baltic and thereby secure the Swedish possessions there.

Finnish troops are conscripted in great numbers into the Swedish army to fight in this or in other wars, and the Finns often distinguish  themselves on the battlefield.

Sweden is the third-largest country in Europe by land area in the middle of the seventeenth century, only surpassed by Russia and Spain.

Sweden reaches its largest territorial extent under the rule of Charles X after the treaty of Roskilde in 1658.

The foundation of Sweden's success during this period is credited to Gustav I's major changes to the Swedish economy in the sixteenth century, and his introduction of Protestantism.

In the seventeenth century, Sweden is engaged in many wars, for example with the Polish–Lithuanian Commonwealth, with both sides competing for territories of today's Baltic states, with the disastrous Battle of Kircholm being one of the highlights.

The Swedes conduct a series of invasions into the Polish–Lithuanian Commonwealth, known as the Deluge.

The szlachta, in its periodic opportunities to fill the throne, exhibits a preference for foreign candidates who will not found another strong dynasty.

This policy produces monarchs who are either totally ineffective or in constant debilitating conflict with the nobility.

Furthermore, aside from notable exceptions such as the able Transylvanian Stefan Batory (r. 1576-86), the kings of alien origin are inclined to subordinate the interests of the commonwealth to those of their own country and ruling house.

This tendency is most obvious in the prolonged military adventures waged by Sigismund III Vasa (r. 1587-1632) against Russia and his native Sweden.

On occasion, these campaigns bring Poland near to conquest of Muscovy and the Baltic coast, but they compound the military burden imposed by the ongoing rivalry with the Turks, and the Swedes and Russians will extract heavy repayment a few decades later.

King Sigismund III Vasa of the Polish-Lithuanian Commonwealth has received military support of five thousand infantry and reiters from emperor Ferdinand II.

Stanisław Koniecpolski, Field Crown Hetman of Poland, leads the Commonwealth army.

Reinforcements, led by Imperial general Hans Georg von Arnim-Boitzenburg and by Ernst Georg Sparr, had arrived in Prussia in late spring 1629 and set up camp near Graudenz (Grudziądz).

Gustav Adolf had arrived in May.

Several skirmishes (recorded as Scharmuetzel) have occurred, one on June 17, 1629 at Honigfeld(t) or Honigfelde near Sztum, where Gustav Adolph leads his army of total of four thousand cavalry and five thousand infantry from Marienburg (Malbork) against the Imperial and Polish forces.

The Swedish king narrowly escapes capture: wounded several times, Gustavus is at one point was saved by one of his men—Eric Soop.

During the battle, a relatively minor affair in eastern Prussia, the Swedish cavalry suffers serious losses, with about six hundred dead and two hundred captured by the Poles, including many high ranking officers.

The Swedish infantry, however, remain mostly intact, so the balance of forces in the war does not change.

The skirmishes continue in July and August and end with stalemate.

The Polish parliament (Sejm) had not imposed new taxes in order to pay the soldiers of the imperial army fighting under Hans Georg von Arnim-Boitzenburg.

Due to low morale some of them had mutinied or gone over to the Swedish side.

Several other countries intervene diplomatically and Sigismund III is eventually forced to enter truce proceedings.

The Truce of Altmark, signed on September 25, 1629 at the Altmark (Stary Targ), near Danzig (Gdańsk) by Sweden and the Polish-Lithuanian Commonwealth during the Thirty Years' War, ends the Polish–Swedish War (1626–1629).

The conditions of the truce allow Sweden to retain control of Livonia and the mouth of the Vistula river.

Sweden also evacuates most of the Duchy of Prussia, but keeps the coastal cities.

the Polish-Lithuanian Commonwealth gets back other Swedish gains since the 1625 invasion.

The greater part of Livonia north of the Western Dvina (Vidzeme) is ceded to Sweden, though Latgale, the southeastern area, remains under Commonwealth rule.

Sweden receives the right to two thirds of all the shipping tolls at ports of the Polish-Lithuanian Commonwealth, such as Danzig and Elbląg (Elbing) and from the Duchy of Prussia ports for the next six years.

These shipping tolls will finance Sweden's involvement in the Thirty Years' War.