Fighting started when the Confederates bombarded Fort Sumter, a Union Army fort. Lincoln called for 75,000 volunteers for a period of 90 days. He also set up a blockade of the Southern coast and ports. Lincoln had no more than 40 usable warships and needed to start building up his Navy. This was to prevent the South from exporting its cotton and importing the war materials they needed to wage war. The North also needed to develop a fleet of gunboats for a "brown-water navy". This was to support the Union Army in operations around major rivers like the Mississippi.