To partition Poland, an agreement between Prussia and Russia alone was not enough; they also needed the recognition of the European powers, especially the understanding of France and Austria.
Vienna Palace
Foreign Minister Wessenberg said, "Your Majesty, we have reached an agreement with Prussia and Russia. They will publicly support our unification of the German lands in exchange for our neutrality on the Polish question."
This was not an empty promise, but it was even less reliable than one. If Austria truly unified the German lands, Prussia and Russia would be the first to react most fiercely.
Before interests, verbal promises had no binding force. Whether they would be honored depended entirely on everyone's sense of decency.
Franz did not believe Prussia and Russia would honor their promise. They might be fighting each other to the death now, but if Austria moved to unify the German lands, the two countries would quickly ease their conflict.
No matter how deep the hatred between Prussia and Russia, could it compare to that between Britain and France? After the rise of the German Empire in the original timeline, Britain and France had been able to cast aside their hatred. Prussia and Russia could do the same now.
In the final analysis, it was still about interests. Austria's unification of the German lands would threaten their security, and for the Kingdom of Prussia in particular, it would be a political blow of near-fatal proportions.
In recent years, the Berlin Government had deliberately downplayed the cause of Greater German unification and promoted the idea of Greater Prussia, precisely because the Junker Nobility feared Austria would breach their fortress from within.
If Austria annexed the German Federal Empire, it would inevitably stir up German nationalism within Prussia. That was something the Berlin Government could never accept.