A second raid was attempted a few weeks later but by this time the British were on the watch. Two of the best German cruisers were sunk and the others barely escaped the fire of the avengers. About the first of June, 1916, a goodly portion of the German fleet sailed out, hoping to catch the British unawares. They were successful in sinking several large ships, but when the main British fleet arrived they began in turn to suffer great losses, and were obliged to retire. With the exception of these two fights and two other battles fought off the coast of South America (in the first of which a small English fleet was destroyed by the Germans, and in the second a larger British fleet took revenge), there have been no battles between the sea forces. The big navy of England ruled the ocean. German merchant vessels were either captured or forced to remain in ports of neutral nations. German commerce was swept from the seas, while ships carrying supplies to France and the British Isles sailed unmolested—for a time. Only in the Baltic Sea was Germany mistress. Commerce from Sweden, Norway, and Denmark was kept up as usual. Across the borders of Holland and Switzerland came great streams of imports. Merchants in these little countries bought, in the markets of the world, apparently for themselves, but really for Germany. However, not for long did British commerce sail unmolested. A new and terrible menace was to appear. This was the submarine boat, the invention of Mr. John Holland, an American, but improved and enlarged by the Germans. In one of the early months of the war three British warships, the _Hogue_, the _Cressy_, and the _Aboukir_, were cruising about, guarding the waters of the North Sea. There was the explosion of a torpedo, and the _Hogue_ began to sink. One of her sister ships rushed in to pick up the crew as they struggled in the water. A second torpedo struck and a second ship was sinking. Nothing daunted by the fate of the other two, the last survivor steamed to the scene of the disaster—the German submarine once more shot its deadly weapon, and three gallant ships with a thousand men had gone down. This startled the world. It was plain that battleships and cruisers were not enough. While England controlled the surface of the sea, there was no way to prevent the coming and going of the German submarine beneath the waters. All naval warfare was changed in a moment; new methods and new weapons had to be employed.