Final answer:
Colonel Smith sent Major Pitcairn ahead likely hoping to retain some element of surprise and to quickly engage the colonists' militia, but the Americans were well-prepared and awaiting the British, thanks to their effective alarm system and advance knowledge of the British movements.
Step-by-step explanation:
Colonel Smith sent Major John Pitcairn ahead to Concord because, despite Pitcairn's efforts to keep their plans from the colonists, they were already aware of the British intentions. George III and his ministers were under the impression, as stated by Pitcairn, that a swift military action might quell the rebellion swiftly.
However, militia and minutemen were prepared for the arrival of the British forces, having already moved munitions out of Concord for safekeeping and being warned by riders like Paul Revere and William Dawes. Smith likely hoped that a quick march by Pitcairn's troops might yet surprise the militia, and at the same time, he called for reinforcements, understanding the perilous position of his troops.