Seven Years%27 War:

Britain now threatened to withdraw its subsidies if Prussia didn't consider offering concessions to secure peace. As the Prussian armies had dwindled to just 60,000 men and with Berlin itself under siege, Frederick's survival was severely threatened. Then on 5 January 1762 the Russian Empress Elizabeth died. Her Prussophile successor, Peter III, at once recalled Russian armies from Berlin (see: the Treaty of Saint Petersburg (1762)) and mediated Frederick's truce with Sweden. He also placed a corps of his own troops under Frederick's command This turn of events has become known as the Miracle of the House of Brandenburg. Frederick was then able to muster a larger army of 120,000 men and concentrate it against Austria. He drove them from much of Saxony, while his brother Henry won a victory in Silesia in the Battle of Freiberg (29 October 1762). At the same time, his Brunswick allies captured the key town of Göttingen and compounded this by taking Cassel.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What was the unexpected action by Peter III that helped Frederick?
He also placed a corps of his own troops under Frederick's command