Answer: Coal
The industry exploded after the war becoming more important to economy
The answer above me is correct
The United States home-front during World War I saw a systematic mobilization of the country's entire population and economy to produce the soldiers, food supplies, ammunition and money necessary to win the war. Although the United States entered the war in April 1917, there had been very little planning, or even recognition of the problems that Great Britain and the other Allies had to solve on their own home fronts. As a result, the level of confusion was high in the first 12 months.
The war came in the midst of the Progressive Era, when efficiency and expertise were highly valued. Therefore, both individual states and the federal government established a multitude of temporary agencies to bring together the expertise necessary to redirect the economy and society into the production of munitions and food needed for the war, as well as the circulation of beliefs and ideals in order to motivate the people.
