Complex sentence: Mahan contended that with a command of the sea, even if local and temporary, naval operations in support of land forces can be of decisive importance and that naval supremacy could be exercised by a transnational consortium acting in defense of a multinational system of free trade.
Split into simpler sentences: Mahan contended that with a command of the sea, even if local and temporary, naval operations in support of land forces could be of decisive importance. He also believed that naval supremacy could be exercised by a transnational consortium acting in defense of a multinational system of free trade.