For each group of belligerents, indeed, the enemy's commerce warfare assumed a vital significance. "No German success on land," declares the conservative British Annual Register for 1919, "could have ruined or even very gravely injured the English-speaking powers. The success of the submarine campaign, on the other hand, would have left the United States isolated and have placed the Berlin Government in a position to dominate most of the rest of the world." "The war is won for us," declared General von Hindenburg on July 2, 1917, "if we can withstand the enemy attacks until the submarine has done its work." Commerce warfare at once involves a third party, the neutral; and it therefore appears desirable, before tracing the progress of this warfare, to outline briefly the principles of international law which, by a slow and tortuous process, have grown up defining the respective rights of neutrals and belligerents in naval war. _Blockade_ is among the most fundamental of these rights accorded to the belligerent, upon the conditions that the blockade shall be limited to enemy ports or coasts, confined within specified limits, and made so effective as to create evident danger to traffic. It assumes control of the sea by the blockading navy, and, before the days of mines and submarines, it was enforced by a cordon of ships off the enemy coast. A blockade stops direct trade or intercourse of any kind. Whether or not a blockade is established, a belligerent has the right to attempt the prevention of _trade in contraband_. A neutral nation is under no obligation whatever to restrain its citizens from engaging in this trade. In preventing it, however, a belligerent warship may stop, visit, and search any merchant vessel on the high seas. If examination of the ship's papers and search show fraud, contraband cargo, offense in respect to blockade, enemy ownership or service, the vessel may be taken as a prize, subject to adjudication in the belligerent's prize courts. The right of merchant vessels to carry defensive armament is well established; but resistance justifies destruction. Under certain circumstances prizes may be destroyed at sea, after removal of the ship's papers and full provision for the safety of passengers and crew.