Treaties of Tilsit
From January to July 1807 the Department was known as the Białystok Department ("Departament Białostocki"), with its capital at Białystok. However, after the Treaties of Tilsit, the Russian Empire agreed to the creation of the Duchy of Warsaw; but in exchange the Department ceded four counties: Białostocki, Bielski, Sokólski, and Drohicki. Thus the Department's capital had to be moved, and its name was accordingly changed to that of the new capital, Łomża.