Answer:
Washington forced British army to leave Boston after he put cannons and fortifications above the city. British army was afraid that Continental army will destroy them and that is why decided to retreat.
Step-by-step explanation:
Fortification were created on Dorchester Heights. As the British soldiers realized that they are in bad position of March 17, 1776 they decided to leave the city. This was one of the first great victories of Continental Army during the Revolution.