By early 1791, around February, Prussian diplomacy managed to secure an agreement with Austria. Although Austria refused to switch sides, it also agreed to remain neutral in case of a war between Russia and Prussia. This convinced Pitt to assure Frederic that Britain would send a fleet to the Baltic, and to craft an ultimatum to Russia, requiring it to retreat from its war with the Ottomans or face an Anglo-Prussian intervention; by late March this position got royal approval and the debate in British parliament was to take place soon. In the meantime, in early March, Frederic Wilhelm II assured an Ottoman representative of his desire to attack Russia as soon as the Royal Navy appeared in the Baltic Sea, and encouraged him, in a handwritten letter, to continue the ongoing war with Russia and undertake a new offensive in the Balkans. Prussia's military preparations were very advanced, with about 90,000 troops massing on the border, and plans for three corps to advance on Riga. Russia itself was gearing itself to defend the Baltic sea border against the expected invasion.