I have seen many letters like this sort, and they all are different, but of course threatening is a great part of it. Like for example this one:
Before Willemstad, 28 February 1793, the second year of the Republic.
I have orders to attack the place you command; the means that I possess to reach this goal, are twofold from what I need, to bring you down, and if these means would be exhausted, they would immediately be replenished by those who are behind me and within my reach. From this sketch, which is not exaggerated, you must understand that all resistance from your side is senseless.
Save yourself the regrets that is being prepared by your foolish obstinacy: open your eyes and take a look at the people and property that is entrusted to you, and try to save them and to act humanly, instead of being bold without results.
I have pity with the wretched situation in which I found the inhabitants of Klundert; they will curse their foolish commander forever, who brought so much misery upon them because of his stubbornness, with results they will suffer for a long time. Beware yourself, my Lord! For the same reproach from those, whose father you must be!
The French are only at war against tyranny: they are the brothers and friends of all humans, and are only aiming at their welfare and happiness. This is the object of all their labour; woe betides the men! who feed the hope, to oppose such laudable objects!
Until now my Lord, I have spoken to you the language of the reason; because I hold you for a wise and human person; but if you, against my expectations, force me to use the power and the means that I have, you will pay with your head surrounded by the ruins and flames, for the disasters that you have caused to happen by your own doing.
When I became master of Klundert, I heard that the commander had sounded the retreat; but the wind and the noise of my batteries prevented me hearing this: this unforeseen event was the cause for even more misfortunes. To prevent the same thing happening between you and me, let us agree that, if we would start a fight against the wish of my heart, and you would like to end it, you can let me know your intentions during daylight by hoisting a white flag, and during the night by lightening three fires next to each other.
I finish my letter my Lord, by telling you that by making this letter known to all, as well as your answer, will justify the disasters that will happen, in the eyes of the reasonable ones, as well in the eyes of the military scientists.
Waiting for the moment to meet you in person, and assured from my deepest regards,
The Marechal de Camp . Commanding the Advance Guard of the Left Army.
P.S. I have the honour to inform you, that today Geertruidenberg and Steenbergen are attacked.
On the other hand, I believe the soldiers that had to storm the city should have been promised something (loot and plunder?), explicit or implicit, to 'raise morale'.
Best regards, Geert