The British also sent a fleet up the Potomac to cut off Washington's water access and threaten the prosperous ports of Alexandria, just downstream of Washington, and Georgetown, just upstream. The mere appearance of the fleet cowed American defenders into fleeing from Fort Warburton without firing a shot, and undefended Alexandria surrendered. The British spent several days looting hundreds of tons of merchandise from city merchants, then turned their attention north to Baltimore, where they hoped to strike a powerful blow against the demoralized Americans. Baltimore was a busy port and was thought by the British to harbor many of the privateers who were raiding British shipping. The British planned a combined operation, with Ross launching a land attack at North Point, and Vice-Admiral Sir Alexander Cochrane laying siege to Fort McHenry, which was the point defensive installation in Baltimore Harbor.