Middle Ages

In the early Middle Ages the area comprising modern Sejny was inhabited by the Yotvingians, one of the Baltic tribes closely related to Lithuanians that had arrived in the area in the first millennium. There was ongoing strife for the area since the 13th century between the Teutonic Order and Lithuania. This resulted in the area being almost completely depopulated, with only a few of the indigenous Yotvingians surviving.^[3] The first written record of the area where the town now lies dates to 1385, noting an armed raid of the German knights from Castrum Leicze (Giżycko) to Merkinė.

After the Treaty of Melno in 1422, Teutonic-Lithuanian border was determined, people began to return to the forests in the area. The territory formed part of the Grand Duchy of Lithuania within the Polish–Lithuanian union under the Jagiellonian dynasty, since 1569 transformed into the Polish–Lithuanian Commonwealth. New roads were paved, with one of them, the road leading from Berżniki (Beržininkai) through Sejny to Merkinė, becoming a notable trade route.

Early modern era

19th century

During the Third Partition of Poland, Sejny was annexed by the Kingdom of Prussia in 1795 and was made part of the newly established province of New East Prussia. Initially neglected, in 1807 the town became part of the short-lived Polish Duchy of Warsaw and a major administrative centre within the Łomża Department. After the defeat of Napoleon Bonaparte in 1815, the town became part of newly formed Congress Poland in the Russian Partition of Poland, retaining its status as a seat of a powiat within the Augustów Voivodeship. In 1818 the bishopric was moved to Sejny from Wigry and Sejny Priest Seminary was established in 1826. The town continued to prosper, despite a major fire that had struck the city earlier that year. The population also grew rapidly. Prosperity declined in the latter part of the 19th century.

World War I

During World War I, in 1915 the town was captured and then occupied by Germany. Initially their forces made it part of the Ober Ost military administration, but Germany planned this area to be one of the puppet states in Central Europe in accordance with their Mitteleuropa plan. Following Germany's defeat in the war, the German garrisons started to withdraw from the area.

Interbellum

On May 8, 1919 Germany handed over the administration to Lithuania which recently declared its independence. This led to tensions between Lithuania and Poland (which also declared independence around the same time), as both claimed the area. Under pressure from the Conference of Ambassadors (later to become the League of Nations), Poland initially backed down on the issue, but, on August 22, 1919, on the day German troops withdrew from the area, Polish Military Organization organized a military action against the Lithuanian rule in what became known as the Sejny Uprising (or "Seinai Revolt"). Sejny changed hands several times. During these battles, both sides used repressive measures - the Lithuanians had deported a lot of Polish families, while the Polish had closed all Lithuanian schools.^[8] On August 28, the uprising ended with a Polish success, and the town became a part of Poland. After Poles acquired the town and its surroundings, the Lithuanian population of the region was subject to various repressions, which included evictions; banning the use of Lithuanian language in public; closures of Lithuanian organizations, schools and press; and confiscation of property.^{[8][9][10][11][12]} The Polish historian Piotr Łossowski has suggested that both sides exaggerated repressions they suffered during the uprising and its aftermath in order to elicit internal and external support.^[13] Sejny town at the time had approximately 2500 inhabitants.^[14]

Only a year later, the town was captured by the Soviet Russia in the course of the Polish–Soviet War. To ensure the right of passage through Lithuanian territory, on 12 July 1920 the Soviet authorities signed the Soviet-Lithuanian Treaty of 1920, in which it ceded the area to Lithuania. On July 19 the Lithuanians recaptured the town. Then the Lithuanians closed Polish schools and offices, and began harassing Poles. After the Battle of Warsaw the Bolshevik forces were defeated and the Polish Army attacked the Lithuanian forces. As the Paris Peace Conference established the Polish-Lithuanian border roughly correspondent to the status quo ante bellum, the Lithuanian forces were forced to withdraw from the town and on 31 August 1920 the town once again passed to Polish hands. However, the Lithuanian authorities still claimed the area and Lithuanian successfully counterattacked on September 2.^[9] However, the Polish Army recaptured the town on September 9. The following day the last of the Lithuanian units retreated to the other side of the border and on October 7 a cease fire agreement was signed, leaving Sejny on the Polish side of the border, so Sejny and the vicinity became part of reborn Polish state. In the interbellum, the town was still desired by Lithuania.

In 1925 the bishopric and the powiat status were removed, but the town remained a notable centre of not only trade and commerce but also wood and furniture production, gaining profits from the nearby forests. In 1925, Sejny became part of the Suwałki County (powiat) and it was only in 1956 that the Sejny County (powiat) was restored (during the Polish People's Republic).

The 1930s saw the development of the town and the construction of new residential buildings and workplaces. Sejny was part of Poland "B", that is, it was located in a poorer part of Poland.

World War II

During the invasion of Poland, which started World War II, in 1939 the town was first captured by the Soviet Union on September 24, 1939. Sejny was severely pillaged and then on 13 October 1939 transferred to Nazi Germany. It remained occupied by the Germans until August 1944.

Poles were subjected to mass arrests, executions and deportations to Nazi concentration camps, and the local Jewish community was almost completely destroyed. The Germans carried out a massacre of 10 Poles at the local cemetery in 1939.^[15] The Germans closed down both Polish and Lithuanian schools and then looted and destroyed school libraries.^[16] Poles managed to organize secret teaching, but soon the Germans carried out mass arrests of Polish teachers, who were then imprisoned in a Gestapo jail in Suwałki.^[16] Nevertheless, secret Polish education continued until the end of the occupation.^[16] In April 1940 the Germans carried out mass arrests of 150 Poles, 10 of whom were publicly executed at the market square on April 24, 1940, and the wójt of Gmina Sejny was murdered in Prudziszki on April 26, 1940.^[17] Several Polish priests were arrested and deported to the Sachsenhausen, Soldau and Dachau concentration camps, where most of them died.^[18] Two priests survived the concentration camps and the war: one, who also held American citizenship, eventually settled in the United States, where he received medical treatment in the following years, and the other returned to Poland after surviving Nazi human experimentation in Dachau.^[19] Poles were expelled from church properties, which were then handed over to Germans as part of the Lebensraum policy.^[20] Works of art and vital records were robbed by the Germans from the local church and taken to Königsberg.^[21]

On 31 August 1944 the town was captured by the Red Army and soon afterwards it was delivered to the new Soviet-controlled communist authorities of Poland.

Recent period and modern Sejny

After the war, the local population, largely depleted during the war, started to recuperate. A notable influx of Poles resettled from the Polish areas annexed by the Soviet Union allowed for fast reconstruction of the town. In 1956, after the administrative reform of that year, Sejny once again became a seat of a powiat. Although it was cancelled in 1975, the new administrative division of Poland passed in 1999 reinstituted it. Depopulation may ultimately again threaten the present local and regional administrative organisation.

Currently, Sejny is a notable centre of trade, production and tourism, with thousands of tourists visiting the town every year. A dairy and a cheese factory are located in the town, as well as numerous hotels.

Sejny is also a notable centre of the cultural life of the Lithuanian minority in Poland. It is the main seat of the Lithuanian Society of Poland and the Aušra bi-weekly. Lithuanians constituted 7.9% (474) of the city's inhabitants in 2002^[22] and 20.2% (4,271) of the population in the Sejny County (powiat) in 2011.^[23] Due to that, there is a Lithuanian consulate there, as well as a Lithuanian schooling complex (kindergarten, elementary school, gymnasium).

Owing to a lost dispute about payment for the town's sewage treatment works, the town is financially crippled by debt. Mayor Arkadiusz Nowalski is fighting to rescue the town, which lacks sufficient collateral for necessary investment or to seek European development funding. Although mayor, with his 2014 shock win against the incumbent, he does not have the majority support of the council.

Twin towns – Sister cities

Sejny is twinned with:

• Anykščiai, Lithuania