Who was Abmiral Alfred Thayer Mahan? What did his book argue?

He was a US naval captain, later admiral, who came into prominence when he published The Influence of Seapower Upon History,1660-1783. In the book and in later writings he argued that to be victorious in war it is necessary to keep sea lanes open to commerce. Conversely the enemy's merchant marine and navies must be neutralized and its ports blockaded. His writings remain very influential and he is still revered in US Navy circles.