In 1812 Clausewitz, with several other Prussian officers, having entered the Russian service, his first appointment was as Aide-de-camp to General Phul.

Afterwards, while serving with Wittgenstein's army, he assisted in negotiating the famous convention of Tauroggen with York.

Of the part he took in that affair he has left an interesting account in his work on the "Russian Campaign." It is there stated that, in order to bring the correspondence which had been carried on with York to a termination in one way or another, the Author was despatched to York's headquarters with two letters, one was from General d'Auvray, the Chief of the Staff of Wittgenstein's army, to General Diebitsch, showing the arrangements made to cut off York's corps from Macdonald (this was necessary in order to give York a plausible excuse for seceding from the French); the other was an intercepted letter from Macdonald to the Duke of Bassano.

With regard to the former of these, the Author says, "it would not have had weight with a man like York, but for a military justification, if the Prussian Court should require one as against the French, it was important." The second letter was calculated at the least to call up in General York's mind all the feelings of bitterness which perhaps for some days past had been diminished by the consciousness of his own behaviour towards the writer.
As the Author entered General York's chamber, the latter called out to him, "Keep off from me; I will have nothing more to do with you; your d----d Cossacks have let a letter of Macdonald's pass through them, which brings me an order to march on Piktrepohnen, in order there to effect our junction.

All doubt is now at an end; your troops do not come up; you are too weak; march I must, and I must excuse myself from further negotiation, which may cost me my head." The Author said that be would make no opposition to all this, but begged for a candle, as he had letters to show the General, and, as the latter seemed still to hesitate, the Author added, "Your Excellency will not surely place me in the embarrassment of departing without having executed my commission." The General ordered candles, and called in Colonel von Roeder, the chief of his staff, from the ante-chamber.