Complex sentence: Another economic slowdown affected the city in 1937, but the city's fortunes rose again during the Second World War; the Frood Mine alone accounted for 40 percent of all the nickel used in Allied artillery production during the war.
Split into simpler sentences: Another economic slowdown affected the city in 1937, but the city's fortunes rose again during the Second World War. The Frood Mine alone accounted for 40 percent of all the nickel used in Allied artillery production during the war.