During World War II the United States attempted to prevent Germany from acquiring wolfram (tungsten ore) in Spain by buying it in the open market. Preemptive buying forced the Germans to pay more for and to consume less tungsten, a material crucial for the production of machine tools and armor-piercing shells. The income provided an important incentive for Spain to remain neutral. The naval blockade and the relatively low elasticity of the supply of wolfram contributed to the success of the wolfram-buying campaign.