Działdowo (; , ) is a town in northern Poland with 20,935 inhabitants as of December 2021, and then the townspeople expelled the Teutonic Knights and recognized the Polish King as rightful ruler. During the subsequent Thirteen Years' War, the town was briefly captured by the Teutonic Knights in 1455. After the peace treaty signed in Toruń in 1466, it became a part of Poland as a fief held by the State of the Teutonic Knights.
Within the Duchy of Prussia (a Polish fiefdom until 1657), the settlement converted to Lutheranism during the Protestant Reformation of the 16th century. The following communes belonged <!-- when ?-->to the Protestant parish of Soldau: Borowo, Bursz, Gajówki, Kisiny, Komorniki, Księży Dwór, Kurki, Malinowo, Pierławki, Prusinowo, Rudolfowo, Wysoka, and Zakrzewo.
Kingdom of Prussia and Germany
thumb|The castle in the 19th century
thumb|Exaltation of the Holy Cross church
In 1701, the town became part of the Kingdom of Prussia, from 1773 on within the newly formed province of East Prussia. Within the Kingdom of Prussia and the later German Empire, the settlement developed into an important Prussian Eastern Railway junction in the second half of the 19th century.
The town had a Polish majority in 1825, with 1,496 Poles and 386 Germans living within it. At the same time, the Prussian authorities were hostile to the local population due to its Polish pro-independence activity during the November and January Uprisings in Congress Poland. During the January Uprising of 1863 an ammunition depot and contact point was secretly established by local people trying to help their fellow Poles in the struggle against the Russian Empire; it was located at the house of Doctor Russendorf. The area remained Polish despite attempts at Germanisation. In 1815, 79% of the local population were Poles, and only 21% Germans. In 1837, 74% were Poles and 26% Germans. In the second half of the 19th century, 87% of the district's population were Poles. With the increase of Germanisation efforts in 1910, the whole district population was divided among 57% Poles, 42% Germans and 1% Jews according to official German statistics. In 1912, the Germans introduced the terms "Masurs" and "Masurian language" instead of "Poles" and "Polish language" in the census in the area. The Prussian Eastern Railway connection to German Prussia such as Deutsch Eylau (Iława), Osterode (Ostróda), and Neidenburg (Nidzica) in East Prussia were severed after the border changes. After the town was ceded to Poland, a large number of the German inhabitants left, including not only German-speakers, but, at roughly the same percentage, Polish-speakers, despite Polish campaigns to win them over as Polish nationals.
The candidate of the German Party, Ernst Barczewski, was elected to the Sejm with 74.6% of votes in 1920, and to the Polish Senate with 34.6% of votes for the Bloc of National Minorities in 1928. However, the city was soon recovered by the Polish Army.
The city became a part of the Warsaw Voivodeship in 1938 after being a part of the Pomeranian Voivodeship for 19 years.
World War II
thumb|Monument to victims of the German Nazi [[Soldau concentration camp]]
During the German Nazi - Russian Soviet invasion of Poland in 1939 that began World War II, the town was invaded by Germany, and then the Einsatzgruppe V entered to commit crimes against the Polish population. Under German occupation, the town was annexed back into the Neidenburg District by Nazi Germany. The German minority in the town formed the Selbstschutz death squad that captured and tortured Polish leaders and members of the political and cultural elites before murdering them. Only some of the local Polish activists were caught by the Germans, as most fled and hid under assumed names in the General Government (German-occupied central Poland).
In 1939, the occupiers established the Dulag Soldau prisoner-of-war camp for captured Polish soldiers at the pre-war Polish military barracks. In December 1939 it was converted into a camp for Polish civilians arrested during the Intelligenzaktion, The Germans also operated two forced labour camps in the town. In 1943 in Warsaw, activists from Działdowo established the secret Masurian Research Institute (Mazurski Instytut Badawczy), which was part of the Polish Secret Teaching Organization. The Poles either returned to Poland later on, or died in Soviet captivity.