The port city was vulnerable to capture by the British by sea. Officials recruited soldiers and studied defenses for invasion from Delaware Bay, but built no forts or other installations. In March 1776 two British frigates began a blockade of the mouth of Delaware Bay; British soldiers were moving south through New Jersey from New York. In December fear of invasion caused half the population to flee the city, including the Continental Congress, which moved to Baltimore. General George Washington pushed back the British advance at the battles of Princeton and Trenton, and the refugees and Congress returned. In September 1777, the British invaded Philadelphia from the south. Washington intercepted them at the Battle of Brandywine but was driven back. Thousands fled north into Pennsylvania and east into New Jersey; Congress moved to Lancaster then to York. British troops marched into the half-empty Philadelphia on September 23 to cheering Loyalist crowds.