Why?
“they left their Command’g officer dying on the ground in his glacis, and retreated with hurry & Confusion/ they rallied in the woods and leaving thear Dead wounded & a few prisoners (which was under the walls of the Fort that could not handely retreat) in all amounting to about three hundred in our hands they returned to Philadelphia that Night”[1]
October 22, 1777 diary entry of Jeremiah Greenman, one of the defenders of the fort. (A significant number of Hessian soldiers also died of wounds during the retreat.) For a description of the attack, see Fort Mercer–Timeline.
When one contemplates the slaughter of the Hessian army besieging Fort Mercer, and the unlikelihood that it could have succeeded in storming the Fort without, at a minimum, ladders, one is left with the question of why the order to storm the Fort was given. Colonel von Donop provided a grandiloquent answer on his deathbed: “I die the victim of my ambition, and of the avarice of my sovereign.”[2] That may be true in a broad sense, but the question of why still bears some consideration.
Why try to take the fort? To start with, it is clear why the British wanted to take Fort Mercer. Although, unlike Fort Mifflin, it was not well situated to fire on ships trying to clear the obstacles in the river, it was a vital supply link to Fort Mifflin. Very likely when the Hessian troops set out from Philadelphia to attack the fort, they had no clear idea of what they faced. While failing to bring scaling ladders seems unforgivable, the British had recent experience with fortified Americans giving up. The fall of Fort Washington in New York was the greatest example of a surrender, while the evacuation of Fort Lee and, more recently and close by, of Fort Billingsport, gave reason to believe that the Hessians might win by just showing up. Also, there was a plan to bombard the fort from the river in combination with the Hessian assault. That did not happen, when the Augusta could not navigate the Billingsport obstructions in time, but had that worked, the Americans would have been under greater pressure to surrender. While von Donop has been criticized for formally demanding surrender instead of making a surprise attack, the thought that the surrender demand, backed by the no quarter promise–i.e., we will kill everyone even if you later give up–if the fort did not surrender, could work was not entirely unreasonable.
Why not just institute a siege? But the Americans refused to surrender. At that point, why not just institute a siege of the fort, using what cannons the Hessians had brought with them to try to destroy the fortifications over time and relying on starvation if nothing else to induce a surrender? There are a couple of possible explanations. There is some suggestion that the Americans hid their strength in such a way as to make their enemies believe the fort was undermanned, so that the possibility of taking the fort by storm may have seemed more reasonable than it turned out to be.[3] The Americans did in fact lack the men to defend the original extent of the fort, and the Hessian forces found it easy to get into that part, which was quickly abandoned and became a killing ground.
An additional factor may have been that the Hessians would be at risk if they tried to stay too long outside the fort on decidedly enemy ground. While in all parts of America there was a split in the populace between patriots and Tories (see Haddonfield), a Hessian force that settled outside the fort for any period of time could quickly find itself surrounded by patriot militia. In fact, New Jersey militia forces helped with the last-minute improvements to the fortifications and were sent away before the attack because there was not room for them in the fort. While the reliability of militia when faced with trained soldiers was always in question, the road back to Boston from Lexington and Concord and the substantial British losses at the battle referred to as Bunker Hill illustrated the damage that militia could do. Also, of course, in time the Continental army under General Washington could show up to deal with a Hessian force that was two days march from the main British and Hessian army in and around Philadelphia.
Why not wait for proper support? The remaining question is of course why not just turn around and go back to Philadelphia, to return later with proper ladders and heavier cannon? Given the relatively recent humiliation of the Hessians in Washington’s Christmas 1776 capture of Trenton, a failure of that type would have been hard for any Hessian commander to stomach. So, given terrible choices, Colonel von Donop made the worst one, for his troops and for himself.
[1] Greenman, J., Diary of a Common Soldier in the American Revolution, 1775-1783: An Annotated Edition of the Military Journal of Jeremiah Greenman, edited by Bray, R. and Bushnell, P., pg. 83 (1978).
[2] M. de Mauduit, who had redesigned the fort and commanded its artillery, led visitors to the site in 1781 and one of those visitors recounts the following, which is attributed directly to de Manduit: Von Donop asked to be told when he was about to die and when informed that his time was near said the following. “it is finishing a noble career early,” said the colonel; “but I die the victim of my ambition, and of the avarice of my sovereign.” Chastellux, F., Travels in North-America : in the years 1780-81-82, pgs. 126-127 (1828), https://www.loc.gov/resource/lhbtn.06665/?sp=81 Given that the quotation was remembered by an eye witness, it seems reasonably likely to have been said. There are those who question whether the reference to the avarice of his sovereign with respect to von Donop’s own death was fair, as von Donop asked to be included in the Hessian troop sent to America, see Huth, H., von Donop, C., et al., “Letters from a Hessian Mercenary,” The Pennsylvania Magazine of History and Biography, Vol. 62, No. 4 (Oct., 1938), pp. 488-501, https://www.jstor.org/stable/20087145 . It does seem fair to attribute his men’s death to the avarice of the sovereign of the small German state from which they came, as he was compensated by England for their fighting against the Americans.
[3] On the other hand, an American officer captured by the Hessians before the assault is said to have exaggerated the number of defenders in an attempt to dissuade the Hessians from attacking. Schenawolf, H., “Battle of Red Bank, October 22, 1777. Incredible American Victory Against Overwhelming Odds,” Revolutionary War Journal, (August 21, 2013), http://www.revolutionarywarjournal.com/battle-of-red-bank/