 On 21 October 1914, the belfry was destroyed, and so was Arras Cathedral on 6 July 1915.In 1917, a series of medieval tunnels beneath the city, linked and greatly expanded by the New Zealand Tunnelling Company, became a decisive factor in the British forces holding the city particularly during that year's Battle of Arras.By the end of World War I, the city was so heavily damaged that three-quarters had to be rebuilt.