in 1817 the then Prussian King was a Calvinist who wanted to have Eucharist with his Lutheran wife(or the other way round), but could not - their churches did not allow that, so he drafted a confessional and set up the Prussian Union. a not very successful amalgamation of Lutheranism and reform traditions.

Basic democratically all the churches in Prussia had to vote in their synods to introduce the mainly liturgical changes in their churches which took a couple of decades, but at least the king could have his Eucharist. So all the churches that are on the territory of the former Prussia are nominally United. But some are still more Lutheran.

The only "important" United Theologians are Schleimacher and Juengle in this century

(Not until the Leuenberger Concordie in 1973 did Reformed, and Lutherans share Eucharist)