The British plan was for the navy, commanded by Vice Adm. Alexander Cochrane, and the army, commanded by Maj. Gen. Robert Ross,
to join in attacking Baltimore. They could not do that until Fort McHenry surrendered, allowing the British ships to get into the harbor.
At Fort McHenry, some 1,000 soldiers under the command of Major George Armistead awaited the British naval bombardment. Their defense was augmented by the sinking of a line of American merchant ships at the adjacent entrance to Baltimore Harbor in order to further thwart the passage of British ships.
The attack began on September 13, as the British fleet of some nineteen ships began pounding the fort with Congreve rockets (from rocket vessel HMS Erebus) and mortar shells (from bomb vessels Terror, Volcano, Meteor, Devastation, and Aetna). After an initial exchange of fire, the British fleet withdrew to just beyond the range of Fort McHenry’s cannons and continued to bombard the American redoubts for the next 25 hours. Although 1,500 to 1,800 cannonballs were launched at the fort, damage was light due to recent fortification that had been completed prior to the battle.
After nightfall, Cochrane ordered a landing to be made by small boats to the shore just west of the fort, away from the harbor opening on which the fort’s defense was concentrated. He hoped that the landing party might slip past Fort McHenry and draw Smith’s army away from the main British land assault on the city’s eastern border. Operating in darkness and in foul weather, Armistead's guns opened fire onto the landing party and the diversionary attack failed. On the morning of September 14, the 30 ft × 42 ft (9.1 m × 13 m) oversized American flag, which had been made a few months before by local flagmaker Mary Pickersgill and her 13-year-old daughter, was raised over Fort McHenry (replacing the tattered storm flag which had flown during battle).
Brooke had been instructed not to attack the American positions around Baltimore unless he was certain they could be taken. Seeing that Cochrane had failed to subdue the fort and that he was heavily outnumbered by the American regulars and militia, Brooke withdrew from his positions, and returned to the fleet which would set sail for New Orleans. Wikipedia
Photos from Fort McHenry Video