Such types of tank units participated in the Soviet invasion of Poland (or, as it is called in Russian historiography, "the liberation march" to West Ukraine and West Belarus), sixteen days after the beginning of the German Invasion of Poland (1939). On 17 September 1939, 878 T-26 tanks of the Belorussian Front (the 22nd, the 25th, the 29th and the 32nd Tank Brigades) and 797 T-26 tanks of the Ukrainian Front (the 26th, the 36th and the 38th Tank Brigades) crossed the Polish border. Combat losses in Poland amounted to 15 T-26 tanks only. However, 302 T-26s suffered technical failures on the march. The Winter War The following tank units, equipped mainly with the T-26, participated in the war with Finland: the 35th, the 39th and the 40th Light Tank Brigades, eight separate tank battalions (OTBs) of rifle divisions of the 8th and the 14th Armies. In the course of the war, the 29th Light Tank Brigade, tank units of the 28th Rifle Corps (four tank regiments, a dozen OTBs of rifle divisions, six separate tank companies of rifle regiments), and five OTBs included into the 9th Army arrived to the front. Light tank brigades in the Winter War were equipped a variety of T-26 tanks, including both twin- and single-turreted tanks produced from 1931 to 1939. Separate tank battalions of rifle divisions had old tanks mainly, produced in 1931–1936. But some tank units were equipped with new T-26 mod. 1939 tanks. A total of 848 T-26s were in tank units of the Leningrad Military District by the beginning of the war. Together with the BT and T-28, the T-26 was part of the main strike force during the breakthrough of the Mannerheim Line, in which tanks shelled antitank teeth, Finnish pillboxes, machine-gun nests, and other fortifications. War experience forced change in the structure of Soviet tank units. T-37 and T-38 amphibious tanks proved to be useless under the conditions found in the northern theatre of operations. In accordance to the letter order of the General Military Council of the RKKA from 1 January 1940, each rifle division should have a tank battalion consisting of 54 T-26 tanks (including 15 flame-throwing tanks) and a rifle regiment should have a tank company of 17 T-26s. The organization of seven tank regiments (164 T-26s in each) for motor rifle and light motorized divisions began at that time also, but only two light