The Americans, having caught word of a British plan to fortify the Charlestown peninsula, decided to get to the peninsula first, fortify it, and present sufficient threat to cause the British to leave Boston. On June 16, 1775, under the leadership of General Putnam and Colonel Prescott, the Americans stole out onto the Charlestown Peninsula with instructions to establish defensive positions on the hills overlooking Boston. A redoubt, a small and usually temporary defensive fortification, was constructed on nearby Breed's Hill, probably due to its closer proximity to Boston compared to Bunker Hill. The next morning, June 17, the British were astonished to see the rebel fortifications upon the hill and set out to reclaim the peninsula. The resulting conflict was called the Battle of Bunker Hill because that is where Prescott originally intended—and was ordered—to build the fortifications. Also, some people considered Breed's Hill a part of Bunker Hill, while others called it Charlestown Hill. British soldiers under Howe sent 2,400 men to attack Breed's Hill. A force of 1,500 colonists held off two British Army attacks, but the third attack, by British Royal Marines, forced the rebels to retreat. 450 colonists were killed or wounded, compared to 1,150 British casualties. In 1825, the Bunker Hill Monument Association began construction of the Bunker Hill Monument, acquiring 15 acres (6.1 ha) of land for the purpose.