The start of the war in Europe is generally held to be 1 September 1939, beginning with the German invasion of Poland; the United Kingdom and France declared war on Germany two days later. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or even the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred simultaneously, and the two wars merged in 1941. This article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935. The British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the forces of Mongolia and the Soviet Union from May to September 1939. The exact date of the war's end is also not universally agreed upon. It was generally accepted at the time that the war ended with the armistice of 14 August 1945 , rather than the formal surrender of Japan, which was on 2 September 1945 that officially ended the war in Asia. A peace treaty with Japan was signed in 1951. A treaty regarding Germany's future allowed the reunification of East and West Germany to take place in 1990 and resolved most post-World War II issues. A formal peace treaty between Japan and the USSR had never been signed.
Question: How many countries were involved in the Battles of Khalkhin Gol?
Answer: 3