Fighting began in Lexington and Concord when British troops

Acaptured Paul Revere.

. came to arrest colonial leaders and take colonial weapons.

Cthought colonial soldiers had fired on them.
Darrived, looking for the Second Continental Congress.

Verified Answer :

Fighting began in Lexington and Concord when British troops "C. thought colonial soldiers had fired on them", although tensions between the colonists and the British had been mounting for quite some time before this.