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.

The word hajduk, which had entered the Polish language from Hungarian in the late sixteenth century, was initially a colloquial term for a style of foot soldier, Hungarian or Turco-Balkan in inspiration, that has formed the backbone of the Polish infantry arm from the 1570s.

Unusually for this period, Polish-Lithuanian hajduks wear uniforms, typically of grey-blue woolen cloth, with red collar and cuffs.

Their principal weapon is a small caliber matchlock firearm, known as an arquebus.

For close combat they also carry a heavy variety of saber, capable of hacking off the heads of enemy pikes and polearms.

Contrary to popular opinion, the small axe they often wear tucked in their belt (not to be confused with the huge half-moon shaped berdysz axe, which is seldom carried by hajduks) is not a combat weapon but a tool for cutting wood.

Michael begins to reestablish domestic order in Russia and seeks peace with Sweden.

After nearly two months of negotiations, representatives from Sweden and Russia meet on February 27, 1617, at the village of Stolbovo (now derelict), south of Lake Ladoga.

Sweden had from the outset entered into negotiations with very high ambitions, with the hopes of fulfilling the old dream of making all Russian trade pass through Swedish territory.

As a consequence of this ambition, the Swedes had originally demanded far-reaching territorial gains into western Russia, including the important northern port of Arkhangelsk.

At this point, however, James I of England had sent a delegation to mediate, as did the Dutch, mostly to make sure Arkhangelsk does not fall into Swedish hands, which would make the extensive trade between Western Europe and Russia far more difficult.

Arkhangelsk does not change hands in the resulting treaty, partly because of the Dutch and English efforts, but mostly because Russia finally has managed to unite under one tsar.

As word reaches Russia that the Swedish war against Poland might soon be over, the Russians are quick to get negotiations going for real —knowing that they cannot afford Sweden's renewal of the war effort on just one front.

In the resulting peace treaty, the Russian tsar and the Swedish king agree to the following terms:
• Sweden gains the province and fortress of Kexholm (Käkisalmi) (now Priozersk) and the province of Ingria
—including the fortress of Nöteborg, known as "the key to Finland" (now Shlisselburg, Russia)
• Members of the upper classes in these conquered areas are allowed to migrate within fourteen days, if they wish to, a right not granted to regular priests and farmers
• Russia renounces all claims to Estonia and Livonia
• Russia will pay Sweden war indemnities of twenty thousand rubles
• Novgorod and other Swedish territorial gains during the war will be returned to Russia
• Sweden has the right to keep all spoils of war collected before November 20, 1616
• The Russian city of Gdov is to remain in Swedish hands until the peace has been confirmed and the borders fully established
• Sweden recognizes Michael Romanov as the rightful tsar of Russia, putting an end to further Swedish claims in Russia
• Russia is allowed free trade at normal trade tariffs, making sure Sweden cannot cripple Russia completely
• Russia is allowed to establish merchant houses in Stockholm, Reval (Tallinn) and Vyborg in exchange for Sweden being allowed to establish merchant houses in Novgorod, Pskov, and Moscow.

Gustavus Adolphus is known to have said about this treaty, which granted Sweden natural borders to Russia, partly in the form of Lake Ladoga and Lake Peipus: jag hoppas att det skall bliva svårt för ryssen att hoppa över den bäcken — "I hope it will be hard for the Russians to jump across that creek".

England is officially credited with brokering this peace, through their mediator John Mericke, though the Dutch efforts are also of great importance.

After the war, the leader of the Dutch delegation, Reinoud van Brederode, is granted the title Baron and given the barony of Wesenberg (Rakvere) in Estonia by Gustavus Adolphus.

Russia will manage to gain back the lost territories as well as to acquire further Swedish lands in the Great Northern War of 1700-1721, putting an end to the Swedish status of a great regional power.

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.