Final answer:
The regulars fired on the militiamen during the Battles of Lexington and Concord as a reaction to an unidentified shot amidst a tense and chaotic situation, not as a result of intentional aggression or a tactical decision.
Step-by-step explanation:
According to Lieutenant John Barker, a British officer present at the Battles of Lexington and Concord, the regulars (British troops) fired on the militiamen due to a chaotic situation, not out of intentional aggression. Barker suggests that after a tense standoff, with both sides in disarray, an unidentified shot ("the shot heard round the world") was all that was needed to set off the tragedy. Although the British troops approached the militia without any intention of injuring them, the sudden gunfire from an unknown source led the nervous and tired soldiers to retaliate defensively and open fire on the militia, resulting in several casualties.