It must have been a relief to General Howe when Washington's army occupied the oddly undefended Dorchester Heights, threatening the city of Boston and the shipping in the harbor with the guns from Ticonderoga. British records show that Howe and his superiors had been advocating for the evacuation of Boston for many months in favor of more significant campaign operations elsewhere: well before Knox's "noble train of artillery" showed up on his doorstep, in fact. It was not their intent to remain in Boston for the next season, as his correspondence with policy makers in London the previous Autumn clearly demonstrates. In fact, had Lord Dartmouth's September 5th, 1775 orders to General Howe not been delayed in crossing the Atlantic, the British might well have gone into winter quarters in Halifax and been long gone from Boston, obviating the need to send Knox to Ticonderoga to dislodge them.
Needless to say, this is not the version of events that Americans remember. We can thank the likes of Sam Adams and a long line of patriotic propagandists and historians for that. The significance of Knox's achievement in bringing the ordinance overland is unquestioningly credited both by his contemporaries and by subsequent writers as the determining factor in forcing the British to abandon Boston. To the degree that negotiations for withdrawal followed swiftly on the heels of the fortification of Dorchester Heights, this conclusion is, at first blush, quite reasonable. General Howe's own account of his decision, written to his superior Lord Dartmouth (who himself had been superseded by Lord Germain in the meantime), appears to lend support to the guns of Dorchester as a causal factor compelling him either to expose the army to the greatest distress or withdraw from Boston. One wonders, however, whether the General did not find in the guns a convenient means for justifying a move he long had contemplated.
Aside from the nightmarish logistical challenges of evacuating the city and its loyal inhabitants, Howe was heavily constrained by the length of time it took to communicate with his superiors, and above all by personal and political considerations. One could not abandoned the city with honor, no matter how prudent or compelling were other military concerns, without having endured (and preferably tried to counter) a proper siege. Until Knox arrived with the cannon, the British may have been on short rations in their winter quarters but they were hardly threatened with reduction, nor unable to leave the harbor so long as there were ships available. The occupation and fortification of Dorchester Heights was a bold stroke, but it also was the key to unlocking the British from a cage of their own making.
Howe did what was expected of him to counter this new threat. He ordered his artillery to fire on the American position, but apparently the guns were unable to elevate sufficiently to hit the heights. One does wonder whether there really were no mortars in the British arsenal capable of doing the job, or whether the attempt was just for form's sake. They evidently spiked and abandoned at least one 13" mortar that had been part of a bomb battery opposing Lechmere. Maybe it was too much to ask to shift that gun to the other side of town, but the idea that the guns wouldn't elevate is clearly a poor excuse.
Michael Pearson's "Those Damned Rebels; The American Revolution as Seen Through British Eyes" describes a British council of war in the aftermath of the aborted night attack subsequently ordered by Howe against the heights:
"Lord Percy, according to engineer Archibald Robinson, advised strongly against persisting with an attack that was likely to be expensive in casualties and could at best only result in controlling a position they were about to abandon.
'Those have been my own sentiments from the first,' said Howe with a sigh, 'but I thought the honour of the troops was concerned.'
It seemed a pretty poor reason for a possible replay of Bunker Hill, and so it evidently appeared to the men sitting at that conference table. For the next morning the evacuation was ordered."
Pearson concludes that "Howe's heart was not in it." This most enigmatic of British commanders during the American War of Independence has been the source of much speculation and second-guessing, and is greatly in need of a clear eyed biography. I am not suggesting that Howe deliberately left the back door open to Boston, offering up his post to the rebels with the sort of perfidy that Arnold intended in betraying West Point. But the honor of their commander was as much in play as the "honour of the troops." Until the conditions for an honorable withdrawal were met, Howe tarried, filling his correspondence with complaints that the season was too advanced, or there were not enough ships to evacuate Boston as his superiors intended and he himself wished.
In this regard, Knox's accomplishment and the overnight fortification of Dorchester Heights are both worthy of American praise and of British relief, for without them, Howe would have waited until the massive reinforcements then being assembled across the Atlantic required his assistance in taking New York - as indeed they did during the coming campaign. In saving face, General Howe gave a boost to patriot arms that they were not to enjoy again until he had swept them from New York and across the Delaware and the Hessians settled into their briefly occupied winter quarters at Trenton.