In the 1930s, the Soviet Union found itself caught in the middle between Germany and Japan, both geographically and militarily. Stalin's focus was on Europe and the rising Nazi tide, but he also worried about the Japanese in the east; Japanese presence in Manchuria posed a threat to his eastern flank. In May 1939, the 129-day border war known as Khalkin Gol broke out. Minor cross-border raids escalated into full-scale clashes between the Soviet, Mongolian and Japanese armies. When, on September 1, 1939, Hitler attacked Poland, an opportunity arose for both sides to disengage, and on September 15, 1939 the war came to a close. The Soviets had lost nearly 8,000 men and suffered over 15,000 wounded. The Japanese, for their part, never officially revealed the scale of their losses, but these were undoubtedly much greater than those of the Soviets. The 129-day bloody border war would lead to an uneasy peace between the two nations from September 1939 until the dying days of World War II (1939-1945).