Peter III was born Karl Peter Ulrich on February 21, 1728, at Kiel in the Duchy of Holstein-Gottorp. He was the son of Charles Frederick, Duke of Holstein-Gottorp, and Anna Petrovna, a daughter of Peter the Great. His mother died shortly after his birth, and his father died when he was eleven. Orphaned and raised by guardians, Peter received a harsh and uneven education focused on military drill and the recovery of Holstein territories from Denmark.

In 1742 his aunt, Empress Elizabeth of Russia, named him her heir and brought him to Saint Petersburg. He converted from Lutheranism to Russian Orthodoxy and took the name Peter Fyodorovich. The transition was difficult. He spoke limited Russian, retained strong affection for German culture, and admired Frederick II of Prussia. Russian courtiers often regarded him as foreign, immature, and insensitive to Orthodox and imperial traditions.

Peter married the German princess Sophie of Anhalt-Zerbst in 1745; she converted to Orthodoxy as Catherine Alexeyevna and later became Catherine the Great. Their marriage was unhappy and marked by estrangement, rival favorites, and disputes concerning their son Paul. Peter devoted much attention to soldiers, uniforms, music, and his Holstein inheritance. His enthusiasm for Prussian military practices intensified as Russia moved diplomatically toward Austria and France.

During the Seven Years’ War, Peter remained heir while Empress Elizabeth’s armies fought the ruler he admired. He followed Frederick’s campaigns and criticized Russian policy, alarming officials who anticipated his accession. This background explains the dramatic reversal that followed Elizabeth’s death. Peter came to the throne in January 1762 with little governing experience, weak support among political elites, and a determination to end the war against Prussia regardless of the sacrifices Russia had already made.