differing speeds caused problems in maintaining formation, while Japanese Nakajima Ki-27 fighters proved adept in exploiting the poor defensive armament of the SB, with the radio operator operating both the dorsal and ventral guns. To minimise losses to Japanese fighters, the Soviets changed tactics, flying SB missions at over 6,100 m (20,000 ft) where it was difficult for the Japanese to intercept. SBs continued to be used against the Japanese as the Soviets and Mongolian forces commanded by Georgy Zhukov carried out a successful offensive until a cease-fire was signed in September 1939. Winter War On 30 November 1939, the Soviet Union attacked Finland in the conflict that became known as the Winter War, with the forces deployed against Finland including several hundred SBs. Losses were heavy, with bomber formations often un-escorted, and forced to operate at low level, where they were vulnerable to Finnish anti-aircraft fire and fighters. While in 1936 in Spain, the SB could outpace enemy fighters, by now it was vulnerable and poorly armed. SBs were fitted with skis for operation from snow covered airfields, slowing the aircraft and making them more vulnerable, while the need to wear heavy winter clothing made the gunner's job even harder. By the end of the 15-week war, at least 100 SBs had been lost, with the Finns claiming nearly 200 shot down, 92 of them to Finnish fighters. Eastern Front When Germany invaded the Soviet Union in June 1941, re-equipment with more modern aircraft such as the Pe-2 had begun. Still, 94% of the Soviet operational bomber force was equipped with SBs, with 1,500–2,000 SBs deployed along the Western border districts of the Soviet Union. The Luftwaffe started Operation Barbarossa with co-ordinated strikes against 66 major Soviet airfields, destroying a large proportion of Soviet air strength on the ground or air on the first day of the invasion. The SBs that survived the carnage of the first day continued to be poorly used, many being frittered away in unescorted low-level attacks against German tanks, where the SB's relatively large size and lack of armour made it highly vulnerable to German light Flak, while German fighters continued to take a heavy toll. Within a few days, losses forced most of the remaining SBs to switch to night attacks. SBs continued to be used, in the defense of Leningrad and Moscow, mainly at night by attacking German artillery. By December 1941