In April 1775 the American Revolution broke out full-scale at Lexington and Concord in Massachusetts. These battles were followed shortly by the convening of the Second Continental Congress in Philadelphia on May 11, 1775. The Philadelphia Committee of Safety, headed by Benjamin Franklin, decided to protect the city by obstructing British access to the Delaware River. Three forts were built to protect two lines of "chevaux de frise" obstacles in the river, designed by Robert Smith. One line was at Fort Billingsport, New Jersey, and another was between Fort Mifflin (called "Fort on Mud Island" or the "Fort Island Battery" at the time) and Fort Mercer. A third line of obstacles was downstream at Marcus Hook with no forts nearby. Forts Mercer and Billingsport in New Jersey were designed by Tadeusz Kościuszko and built as earthworks; work also resumed on Fort Mifflin and all three forts were garrisoned during 1777. Meanwhile, the Continental Congress relocated to Baltimore in early 1776 due to a threat of British attack, later returning to Philadelphia and issuing the Declaration of Independence on July 4, 1776, attracting further British attention to the city.