Realizing that war was imminent, Prussia preemptively struck Saxony and quickly overran it. The result caused uproar across Europe. Because of Prussia's alliance with Britain, Austria formed an alliance with France, seeing an opportunity to recapture Silesia, which had been lost in a previous war. Reluctantly, by following the imperial diet, most of the states of the empire joined Austria's cause. The Anglo-Prussian alliance was joined by smaller German states (especially Hanover). Sweden, fearing Prussia's expansionist tendencies, went to war in 1757 to protect its Baltic dominions, seeing its chance when virtually all of Europe opposed Prussia. Spain, bound by the Pacte de Famille, intervened on behalf of France and together they launched an utterly unsuccessful invasion of Portugal in 1762. The Russian Empire was originally aligned with Austria, fearing Prussia's ambition on the Polish-Lithuanian Commonwealth, but switched sides upon the succession of Tsar Peter III in 1762.