The beginnings of Łomża city, which guards the crossing of the River Narew, dates back to the 10th century. In the early Middle Age the area of two fortified boroughs were inhabited by numerous merchants and craftsmen. The remains of the old settlement, which exists to this day, is Góra Królowej Bony (Queen Bona’s Mountain), with preserved fragments of earth embankments. The earliest references to the Łomża settlement date back to 1400. It is believed that it was established probably around 1390, at a distance of 5 kilometers north-west to the old castle. In 1418, Łomża received the Chełmno law from Prince Janusz I, who was then the ruler of the north-eastern part of Mazovia. Łomża was developing as an administrative and judicial centre, and also as the capital of the Łomża land. Land courts as well as courts of first instance (that took care of cases concerning nobles – TN), and also local nobility assemblies took place there. Around 1478, to the north-west of the city, the so-called new city with a separate market were installed.
In 1562, after the Masovia was joined to the Crown of the Kingdom of Poland, Łomża became a royal city (capital of Łomża land in Masovian Province), which was developing as an administrative and judicial, and also commercial centre. Local merchants took the lead in international trade of wood, corn, salt, flax, tallow and hemp. At least from 1494 on, Jews was present there, and from around 1530 on, there were few Protestants. As a result of deepening economic conflicts between Jews and Christians in the city, in 1556 Łomża received the de non tolerandis Judaeis privilege from king Zygmunt II August. It led to the expulsion of the Jewish population for more than 200 years.
The end of heyday of Łomża was the result of wars and epidemics in the second half of the 17th century, especially the almost total destruction during the Swedish invasion in 1656. At the end of the century, destroyed Łomża was inhabited by barely about 300 people, most of whom were farmers. Although the city was formally the local administrative and judicial centre, during the 18th century it was increasingly loosing importance. In 1791, Łomża was inhabited by 1,000 citizens and instead of three traditional fairs, only one was organized.
As a result of the third partition of the Republic of Poland, Łomża was incorporated into the Prussian Partition. The city became the seat of the county government in Białystok Department of New East Prussia. It was a period of a slow return to its former state after the collapse, which was connected, for example, with re-settling, despite the prohibition that was still in force against Jewish merchants and craftsmen. Also officials and colonists from Prussia, who belonged to the Evangelical Church of the Augsburg confession, settled in the city. During the era of the Duchy of Warsaw between 1807 and 1813, Łomża was the capital of the department, and at the same time the place were Polish troops were stationed.
During the era of the Kingdom of Poland, under Russian rule, Łomża, which was a county town, carried out the function of a local seat of the administrative and judicial authorities. It was also the local centre of trade, industry and services. In the second half of the 19th century, Łomża began to build its position as an industrial centre. It was possible thanks to the location of Łomża next to the road from Warsaw to St. Petersburg through Kowno. In the years 1840-1865, 10 new plants were established, including a few brickyards and a sugar mill. The fast development of the city began after 1866, when Łomża became the capital of the governorate (Polish: gubernia) and the seat of the Russian military garrison, as a result of an administrative reform. The city was extended in a significantly fast pace. The number of residents was also increasing, from 3,000 in 1825 to 26,000 in 1913. Further development of the city was possible thanks to creating a rail connection between Łomża, Ostrołęka and Białystok (through Śniadowo) in 1915.
After the end of World War I, the ethnic structure changed significantly. After Orthodox Russians and German Protestants had left Łomża, it became a city of two religions, being inhabited by Catholic Poles and Jews. After regaining independence, Łomża reached the status of a county town in Białystok Province (from 1939 on in Warsaw Province). It was also the local centre of trade, crafts and industry, and also the seat of garrison at the same time. A few high school operated there, and what is more, the local press in Polish and Yiddish language was published. In 1925, Pope Pius XI made Łomża the capital of Łomża Diocese.
After the outbreak of World War II, on 7 September 1939, Łomża was destroyed as a result of bombing. Three days later Germans entered the city. On 28 September 1939, the city was handed over to Red Army units. Łomża was incorporated into the Byelorussian Soviet Socialist Republic. The occupiers transported to Siberia. When the war between the Germans and the Soviets began, on 22 June 1941, Łomża was bombarded by German Luftwaffe, while on 24 June – occupied by Wehrmacht. In July, the city and the whole land of Białystok were subordinated to the Gauleiter of East Prussia. A ghetto was formed in August. Jewish inhabitants and refugees from other areas were relocated there. In September 1941 about 31,000 Jews from the ghetto were sent before a firing squad; most of those who remained were killed in Treblinka and Auschwitz-Birkenau.
During the war (fights on the line of the River Narew), in winter 1944/1945, about 70 per cent of Łomża's buildings were destroyed. The reconstructed city was the centre of county in Białystok Province to 1975, and next it was the capital of Łomża Province, existing to 1998, as a result of an administrative reform in 1975. In 2013, the city had a status of the centre of Łomża County and a city with county rights (so-called municipal county).
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