"During the civil war the naval contribution to the parliamentary cause was secondary. Victory was decided on land. The fact that Parliament had control of the navy was none the less vital in making victory possible. If the king had retained control of the fleet the royalists could have blockaded London, and the resulting economic dislocation might easily have generated enough popular pressure to force Parliament into peace on almost any terms. During the war the navy's undramatic work in protecting commerce kept up the level of customs revenues and helped finance the war-effort. The navy was an effective deterrent to any foreign monarch tempted to send help to Charles. It assisted land campaigns by transporting supplies and reinforcements and by providing mobile artillery. It played an important role in maintaining the outposts at Hull and Plymouth, and contributed to the capture of Bristol and Newcastle. The earl of Warwick, as Lord High Admiral, and his vice-admiral and successor William Batten provided vigorous and effective leadership."