How did Lexington and concord start?

On the night of Apr. 18, 1775, General Thomas Gage, the British Chief Commander in America sent a detachment of 700 men under Lt. Col. Francis Smith to destroy the military depot the Patriots had established at Concord.

When the British column reached Lexington at dawn of 19th, they found 70 armed minutemen deployed near the village common. Without having been ordered to, the British opened fire, killing eight and wounding ten of the militiamen. The Patriots returned the fire wounding 1 British soldier.