After the fall of France in June 1940, the Free French had only a 3,000 strong contingent in Britain, growing to 7,000 by the end of the year, Poland evacuated around 19,000 to 35,000. By the end of 1940, I Corps in the West (Poland) numbered about 14,000; Polish forces in the Middle East, about 3,000; this does not count the Polish air crews (numbering at least 4,000) and the Polish Navy personnel. After the fall of France, the French forces lagged behind the Polish in numbers. It was only after D-Day and the liberation of the French mainland that French forces swelled to 550,000, outnumbering the Polish Army in the West, but not the combined West, East and partisan forces. Until 1944, Polish forces also outnumbered the French. In 1942, the French resistance numbered about 10,000, (the size of Polish resistance is discussed in note b below) and in 1943, the Free French numbered about 70,000. With the entrance of Soviet Union into the war in June 1941, Poland returned to being the third biggest Ally again, and with the entry of United States in December 41, the fourth. However, the Japanese involvement also marked the connection of the European and African theaters to Second Sino-Japanese War, and estimates cited above ignore China, whose armies totaled about two million by the end of the war. Thus for about a year, Poland could be seen as the second biggest ally, after Britain. It was then superseded by China, the Soviet Union and the United States. Counting China, from the end of 1941, Poland was the 5th biggest ally. Near the end of the war, Polish contribution, in terms of numbers was matched or surpassed by that of France.
Question: How many months passed between the Soviet Union joining the war and the US joining?
Answer: 6