 In the 20th Century the county saw significant growth with the entrenchment of the Dow Chemical Corporation and the establishment of major roads, highways, and a few minor airports.However, in the late 20th Century the Pere Marquette Railroad was closed, which cut off passenger train travel from the region.In World War 1, once the US had entered the war in 1917, the Dow Chemical Corporation, based mainly in the city of Midland, was involved with the production of poisonous gases for use during the war, and with the production of ammunition.