The Powder Alarm was a massive popular reaction to the removal of gunpowder from a magazine by British soldiers under orders from General Thomas Gage, royal governor of the Province of Massachusetts Bay, on September 1, 1774. In response to this action, amid rumors that blood had been shed, alarm spread through the countryside as far as Connecticut and beyond, and American Patriots sprang into action, fearing that war was at hand.
Although it proved to be a false alarm, the Powder Alarm caused political and military leaders to proceed more carefully in the days ahead, and essentially provided a "dress rehearsal" for the Battles of Lexington and Concord seven and a half months later. Furthermore, actions on both sides to control weaponry, gunpowder, and other military supplies became more contentious, as the British sought to bring military stores more directly under their control, and the Patriot colonists sought to acquire them for their own use.
On August 31, Gage sent Middlesex County sheriff David Phips to Brattle with orders to remove the provincial powder; Brattle turned the key to the powderhouse over to Phips. Gage also gave orders to ready a force of troops for action the next day, something that did not go unnoticed by the local population. At some point that day, General Gage, whether by his intent, accident, or theft by a messenger, lost possession of William Brattle's letter; the widely held story is that it was dropped. News of its content eventually spread rapidly, and many considered it to be a warning to Gage to remove the provincial powder before Patriots could seize it.
Early in the morning of September 1, a force of roughly 260 British regulars from the 4th Regiment, under the command of Lieutenant Colonel George Maddison, were rowed in secrecy up the Mystic River from Boston to a landing point near Winter Hill in modern-day Somerville. From there they marched about a mile to the Powder House, a gunpowder magazine that held the largest supply of gunpowder in Massachusetts. Phips gave the King's Troops the keys to the building, and after sunrise they removed all of the gunpowder. Most of the regulars then returned to Boston the way they had come, but a small contingent marched to Cambridge, removed two field pieces, and took them to Boston by foot over the Great Bridge and up Boston Neck. The field pieces and powder were then taken from Boston to the British stronghold on Castle Island, then known as Castle William (renamed Fort Independence in 1779).
Rumors flew throughout the day across the countryside about the British troop movements. The regulars were marching; provincial powder had been seized; war was at hand; people had been killed; Boston was being bombarded by His Majesty's warships. The alarm spread as far as Connecticut. From all over the region, people took up arms and began streaming toward Boston. One traveler in Shrewsbury reported that in the space of 15 minutes, 50 men had gathered, equipped themselves, sent out messengers to surrounding towns, and left for Boston. On the 2nd, several thousand men bent on violence gathered in Cambridge, where they forced several notable Loyalists, including William Brattle, to flee to Boston and the protection of the military. Sheriff Phips was forced, in writing, to dissociate himself from any and all government actions. Eventually facts caught up with the rumors, and militia units (some of which were still heading toward Boston) returned home.
Also on the 2nd, Boston newspapers published a letter from William Brattle in which he protested that he had not warned Gage to remove the powder; Gage had requested from him an accounting of the storehouse's contents, and he had complied. The content of his letter to Gage would be published on the 5th. Brattle remained on Castle Island through the siege of Boston, leaving when the British evacuated the city in March 1776. He died in Halifax, Nova Scotia in October 1776 at the age of 70.
After the Powder Alarm, militia forces throughout New England were more cautious with their supplies and more intent on gaining information about Gage's plans and troop movements. Paul Revere played a significant role in distributing this information due to his geographical position in Boston, his social position as a middle-class craftsman in contact with all social classes, and his political position as a well-known Patriot propagandist and organizer.