(Male) Qualifier added because there's also a Marwitz (Female) Affair involving Hohenzollern siblings.
The documented facts are these:
a) Diary entry by Lehndorff from 1756, giving the following summary:Morning with the King. All are delighted to see our sovereign, and one would adore him if only this great man were a bit more gracious to those who want to adore him. But nothing is more humiliating than having to stand around and to wait for hours to at last see someone who doesn't grace us with a look. The fear which princes inspire only signifies their power. Awe is inspired by their dignity; their true glory springs from the estimation and personal respect one has for them. Friedrich does enjoy this precious advantage, and he would be loved, too, if only he numbered kindness among his qualities.
I renew my acquaitance with a man I had not seen since the year 1749. It is a young Marwitz, who started his career as page with the King, and who became a favourite with him as well as with Prince Heinrich. This affection went so far that the two royal brothers turned incredibly furious on each other for his sake. The young page was sent away, but due to urgent pleadings on Prince Heinrich's side, he got a commission in the guard. Some time later, the Prince accused him of falsehood and bad manners, and banished him completely from his company. Since then, the King has occasionally favoured him with his grace, but in the next moment sends him to guard duty and treats him like a criminal. This man now resurfaces on the horizon; the Prince tells me that he is quite amiable, that he invites him to his parties again, and the King has made him his batman. He posseses wit and is somewhat strongly fantastical; I consider him malicious.
In the evening, the whole royal family dines with the Queen Mother.
b) Letters from Fritz to younger brother Heinrich, written in March 1746:Heinrich got sick near the end of carnival time and thus is in Berlin, when Fritz (who is in Potsdam) writes to him on March 3rd, 1746:
"I am glad to hear you are recovering from your colic. Don't go out again too early, and allow your body time to recover. Your little favourite is doing very well, and if he remains good, you'll soon see him again. Right now, he's pining for love and is composing elegies full of hot kisses in your honor which he intends to give you upon your return. I advise you not to exhaust yourself so that you have enough strength to express your love. The happiness of the immortals will not be equal to yours, and you will be able to drink rivers of lust in the arms of your beloved.
Adieu, mon cher Henri. I hope your illness will be the last with which you will worry my friendship for you, and that I shall soon be able to enjoy your amiable company without having to worry about you. This is still sounds like more or less good natured big brotherly teasing (for Fritz). The next letter, alas, does not. It's dated on March 6th, 1756.
My dear Heinrich, no, there is no crueller martyrdom than separation! How to live for a moment without the one you love? (...) Our sighs travel on country roads, and we pour our heart out as messages of our unhappy souls flying away like doves. Oh! Oh! The faithless man has forgotten me! says a certain person. Already a day has passed without a sigh of his has reached me! Surely, he's become faithless! He doesn't love me anymore! No, he doesn't love me anymore! If I had the courage, I'd tell this charming sad person: "That's no more than you deserve, you damned whore! Didn't you want to infect my poor brother with your gonorhoe? Oh! If he listened to me, he'd turn his love towards a worthier object and would send you to hell with all your nice little qualities, of which your STD, your vanity, your lies and your recklessness are but the least.
I do apologize for having committed the sacrilege of having dared to speak so dismissively of your angel's qualities. I do hope you'll forgive me.Whatever Heinrich replied, Fritz was still not done, and wrote again the next day, March 7th:
There is little more admirable than your fidelity. Since Pharamon and Rosamunde, Cyrus and Mandone, Pierre de Provence and the beautiful Madlone one hasn't seen the like. If you'll allow me, I'll write a novel titled "Fidelity. Love. Henri and the beautiful Marwitz", and it would be a novel so delicate, so tender, so sentimental and so sensual that it would be instructive to our youth. I would paint the gonorhea-ridden Marwitz in such lovely colors, I'd equip him with all the wit he believes himself to have, and I would above all describe all his coy affectations, as far as I was able to, with which he seems to signal silently to everyone: 'Look at me, am I not a pretty boy? Doesn't everyone have to love me, adore me, worship me? What, you little villain, you resist? You haven't yet put your heart at my feet? As for you, my angel, you'll have to die of love for me.'
Afterwards, I must describe the details of his figure, the charm of his wide shoulders, his supposedly heavy but actually seductive walk - in a word - but I can't continue, for otherwise my novel will be written by someone else. To you, my dear Heinrich, I reccommend to eat a lot, drink a lot and sleep a lot. Stay for some more days in Berlin, and do justice to my tenderness for you. Again, we don't have Heinrich's reply. Fritz sounds a bit more apologetic and tries to pass it off as fraternal teasing in the last letter relating to this affair, dated March 9th:
I do hope, my dear Heinrich, that this explanation will mollify you. I haven't said anything detrimental regarding your fidelity. I only listed the famously faithful couples known in history, with whom, incidentally, you can't really compare yourself, for your separation has lasted only ten days so far, and your little sweetheart lives only four miles away from you. Moreover, you can be sure to see him again soon. Pharamon had to wait for ten years before seeing Rosamonde again. I dare say there's a difference. I do hope, dear Heinrich, that this silliness don't rob me of your friendship, and that you will do me more justice in the future. But don't demand me of me that I should take your little romance seriously, and don't sulk over my jokes regarding a matter which wasn't an insult. Adieu, mon cher Henri, and believe me, I didn't hurt you intentionally.c) The Wartime Diary of Heinrich's AD Victor Amadeus Henckel von Donnersmarck covering the years 1756 - 1758. These were later published by Henckel von Donnersmarck's grandson. Notes Henckel on page 220, June 5th 1757:
"On the 5th, the King sent his Quartermaster-Lieutenant and AD, Hauptmann Marwitz, with two Saxon Regiments to Colonel Meier who was camping outside of Nuremberg in order to help him. However, the rumor spread that not this but to go to the Duke of Bevern had been his true mission. This Marwitz had played several roles in his life. He'd started as a page of the King and had sometimes been in favour, sometimes in disgrace. The King had lowered himself to teach him himself, had given him his own books and works to use, and had even comissioned him to write his history."The German phrasing doesn't make it clear whether "his history" means the King's history or Marwitz' history. Also, Henckel still doesn't grace us with a first Name for Marwitz. And note that as opposed to Lehndorff, he seems not to be informed about the romantic history Marwitz has with Heinrich. (Which probably says something about the different types of relationships Heinrich has with Lehndorff and Henckel respectively.) However, by providing us with the information that in-and-out-of-favour former page Marwitz ended up as Quartermaster in the Seven-Years-War, he allows us to identify Marwitz the former page with the Marwitz honored by Heinrich at the Obelisk he built in Rheinsberg, in memory of his brother August Wilhelm as well as 27 other men whom he felt to have been wronged by his brother Friedrich. This is what the inscription at the Obelisk says about Marwitz the quartermaster:
von Marwitz, quartermaster of the King's army. Earned great merits in all wars, was present in all battles and distinguished himself in several incidents. He died in 1759, at the age of thirty-six. Perhaps his value and merits would be forgotten if this monument did not honor his memory. (All the Rheinsberg Obelisk transcriptions are available in German
here.)
d) "Die Pagen am Brandenburg-Peußischem Hofe, 1415 - 1895", a book published in 1895 by a gentleman named von Scharfenorf, Captain A.D. , librarian and teacher at the Cadet Academy, which tells the story of the pages at the Hohenzollern courts for the centuries advertised in the title, and which uses, among other things, the detailed accounts of the Fredersdorf-as-treasurer era of Fritz' court as source material, offers two references to a page who could be "our" Marwitz. In 1742, the page G.W. von der Marwitz hands out money to the poor on royal command after the troop parade at Neisse in the church courtyard (the money in question is four Taler eight Groschen). In 1746, a page von der Marwitz, no initials provided this time in the book, is listed as receiving 66 Taler "Abreisegeld", which technically could mean either travel expenses or severance pay, though since "Abreise", as opposed to "Reise", means "departure", not "journey", I'm tempted to go with the later. (1746 as the year is significant because Fritz' letters to Heinrich referencing "Marwitz" are thankfully dated, see above.)
e) This letter from Fritz to Heinrich dated July 8th 1759 contains a single sentence mentioning Marwitz, but this one highly significant:
Marwitz vient de mourir à Landshut d'une fièvre chaude mêlée de rougeole, "Marwitz has died in Landshut of a hot fever caused by measles".
1759 is the year Quartermaster Marwitz from the Rheinsberg Obelisk inscription has died, so this definitely is the same person. Now the Marwitz family is still large enough that it's possible that Fritz had more than one page of that name in the 1740s who was in and out of favor, and that the one who died in 1759 doesn't have to be the same Marwitz mentioned in the 1746 letters. However, it's worth pointing out that there were several members of the Marwitz clan serving in the 7 Years War - including, for example, the one who will later refuse to sack Hubertsburg -, and yet Fritz does not consider it necessary to tell Heinrich which Marwitz he means in his 1759 remark; he takes it for granted that Heinrich will know whom he's talking about. Which would make sense if this Marwitz had personal meaning beyond other members of his family to both brothers. Droysen, who edited the "Political Correspondence" in which this letter is included in the 19th century, identifies the Marwitz who died as Georg Wilhelm von der Marwitz in the personal register.
f.) As of March 30th 2021, wikipedia has an
entry identifying Marwitz the page/quartermaster as Georg Wilhelm von der Marwitz. The references in the footnotes of this new Wikipedia entry are those sources we've listed above. There's still a margin of error possible - for example, we don't know how Droysen made the identification of the Marwitz who died in July 1759 as Georg Wilhelm - but it does look extremely likely know that all these references, from page G.W. von der Marwitz who has to hand out money to the poor in 1742 to Quartermaster Marwitz from the Rheinsberg Obelisk inscription are the same person and that person featured in a triangle with Friedrich and Heinrich.