One of Gen. Mitchell's most adamant points about aerial bombing was its ability to render a surface fleet vulnerable to attack from the air from both land and carrier based aircraft, thus making it obsolete for use in modern warfare. American military leaders openly scoffed at Mitchell's ideas until his air squadron of Martin Bombers sank the Ostfriesland, a captured German Battleship and one of the most formidable in the world. To accentuate his point, Mitchell later sank a German U-boat and several obsolete American ships. Although successful, the military refused to accept Mitchell's findings.