on april 16, union forces probed a point in the confederate line at dam no. 1. the union failed to exploit the initial success of this attack, however. this lost opportunity held up mcclellan for two additional weeks while he tried to convince the u.s. navy to bypass the confederates' big guns at yorktown and gloucester point and ascend the york river to west point and outflank the warwick line. mcclellan planned a massive bombardment for dawn on may 5. but the confederate army had slipped away during the night of may 3 toward williamsburg.