By early March 1776, heavy cannons that had been captured at Fort Ticonderoga were moved to Boston, a difficult feat engineered by Henry Knox. When the guns were placed on Dorchester Heights in the course of one day, overlooking the British positions, the British situation became untenable. While General Howe planned an attack to reclaim the high ground, a snowstorm prevented its execution. The British, after threatening to burn the city if their departure was hindered, evacuated the city on March 17, 1776 and sailed for temporary refuge in Halifax, Nova Scotia. The local militias dispersed and, in April, General Washington took most of the Continental Army to fortify New York City and the start of the New York and New Jersey campaign.