1928. After that he returned to active service in line units and served in a number of detachments based in Wilno and Modlin, among them the Wilno Fortified Camp (since 1932). Between 1937 and 1938 he served as the commanding officer of the 8th Infantry Division and then the garrison of the Modlin Fortress. As one of the more skilled Polish officers of the time, on 19 March 1939 he was promoted to the rank of brigadier general and became the commander of both the prestigious Polish 1st Legions Infantry Division and the Wyszków Operational Group formed around it. After the outbreak of the Invasion of Poland in 1939, during the opening stages of World War II, Kowalski proved to be one of the most successful commanding officers in the Polish Army at that time. The units under his command entered in contact with the enemy on 4 September in the forests around Długosiodło, to the north of Warsaw. Delaying the German forces in a number of skirmishes and battles along the Narew and near Różan, Kowalski's units managed to retain most of their combat readiness. After the battle for Pułtusk, on 7 September, the Poles were outnumbered 3:1 and ordered to retreat southwards. On the road Gen. Kowalski managed not only to withdraw most of his forces, but also to rally the defeated forces of Modlin Army and Independent Operational Group Narew crowded near the bridge in Wyszków. Thanks to Kowalski's actions, overnight 2 divisions and (33rd Infantry and 41st Infantry), as well as the Mazowiecka Cavalry Brigade were not only rallied but also safely transported to the other side of the Bug River. Kowalski then withdrew with his forces to the area between Wyszków and Kamieńczyk, and organized a successful defence of the line there. After repelling a German assault on Brańszczyk, his forces started to slowly move southwards while performing delaying actions and keeping the combat readiness almost intact. After the German forces seized the town of Kałuszyn thus cutting out Kowalski's men from the safe passage towards the Romanian Bridgehead, on 13 September the division broke through enemy lines and retook the city in what became known as the battle of Kałuszyn. Despite heavy losses on the Polish side, Wincenty Kowalski managed to yet again rally a large part of his forces and continued his move towards Włodawa, Lublin and Lwów. Near Chełm, on September 18