The Polish issue was one of the more contentious ones of the Congress of Vienna and as such it has a tendency to obscure some of the other areas of negotiation. One of the initial goals of Prussia's diplomats at the Congress was to have Prussia absorb the whole of the Kingdom of Saxony and spent a considerable amount of effort to achieve this goal. Neither Britain nor Austria was willing to countenance a total Saxon partition, but this gave both Russia and France a degree of latitude for their own agendas by remaining on the fence. For Russia's part, Emperor Alexander I desired a restoration of Poland under Romanov tutelage. Such an entity not only would have been composed of Russian Poland, but also parts of Prussia and Austria's bounty from the partitions. This enlarged Polish entity would, in Alexander I's calculations, show the Poles that he was a firmer champion of Polish rights than either Austria or Prussia and even the departed Napoleon, whose Duchy of Warsaw never satiated Polish nationalists' aspirations. Alexander I played both sides of the Saxon issue during the opening rounds of the Congress. In the summer and autumn of 1814 he offered Prussia a degree of support over Saxon partition in exchange for a freer hand in Polish boundaries while simultaneously incorporating Saxon volunteers into his corps of body guards during the celebrations of the Battle of Leipzig anniversary.

In the winter of 1814/15, Prussia's diplomats found themselves diplomatically outfoxed on the Saxon issue. Metternich had proposed Prussian minister Karl August von Hardenberg in October 1814 that Austria would back a Saxon partition sans a rump Saxony around Dresden and the Bund's control of the Mainz fortress in exchange for Prussian support of against a reborn Polish state. Von Hardenberg was incensed by Metternich's offer, but the October letter and subsequent negotiations gave Alexander I a pretext to dress down both Prussian King Frederick William III and his chief minister on 5 November and Alexander I falsely claimed that Metternich had promised Russia support over Poland in exchange for the preservation of Saxony. In the meantime, Metternich had managed to secure both France and Britain's support over the Saxon-Poland issue. Alexander I's sabre-rattling over Poland made it appear to some contemporaries that war was in the offing in early 1815 and both Alexander I's and Metternich's machinations meant Prussia was diplomatically isolated. Thus as this crisis in inter-Allied relations died down in the spring of 1815, Prussia had to scramble for whatever favorable concessions it could achieve.

For Prussian statesmen, this meant both accepting a truncated partition of Saxony and the continued existence of the Wettin monarchy south of Prussia and trying to have the most defensible borders against what would soon be known as Congress Poland. This meant giving up the earlier partition lines, which Prussia had already agreed to the previous year in the hopes of gaining Saxony. The spectre of a resurgent Russian-backed Polish nationalism also played a role in Prussia's acceptance of losing some of the Third Partition territory. Johann Gottfried Hoffmann, one of von Hardenberg's chief advisors, noted in Prussian memos that the Poles in West Prussia and Posen could be gradually Germanized under enlightened Prussian administration which was part of a larger Austro-Prussian process to make the Polish population of their territories less "Polish" and more regional in their loyalties.

In light of the Saxon issue, it is not that Prussia did not try to get the Third Partition territories back, but rather Prussian statesmen ceded them partly in hard recognition that the Russians were going to get them anyway but also in the hopes that such a concession would give them leverage to absorb Saxony. Frederick William III's diplomats were correct on the first count, but badly miscalculated on the second. As a result, Prussia had to settle for lesser Saxon gains and non-contiguous territory on the Rhine.

Sources

Kraehe, Enno E. Metternich's German Policy. Volume II. Princeton: Princeton University Press, 1983.

Vick, Brian E. The Congress of Vienna : Power and Politics After Napoleon. Cambridge: Harvard University Press, 2014.