Peter became a Russian tsar in 1682, initially sharing the title with his half-brother Ivan V while their sister Sophia exercised the regency. He displaced Sophia in 1689 and became sole ruler after Ivan's death in 1696. Military experimentation, shipbuilding and contact with foreign specialists shaped his approach to government. Campaigns against the Ottoman Empire secured Azov in 1696, encouraging his ambition to establish a navy and obtain access to international sea routes. On the Great Embassy of 1697–1698 he travelled in western Europe to seek allies and learn about naval and industrial techniques.

Peter redirected Russia's principal military effort toward Sweden in the Great Northern War, which began in 1700. Defeat at Narva exposed weaknesses in the army, but he rebuilt its organization, expanded armaments production and imposed heavy demands for recruits and revenue. In 1703 he founded St Petersburg on territory seized near the Baltic, making it both a strategic port and a new centre of royal power. His victory over Charles XII at Poltava in 1709 changed the balance of the war. The Treaty of Nystad in 1721 confirmed major Baltic acquisitions, and Peter adopted the title emperor. A separate Ottoman campaign ended badly in 1711 and cost him Azov.

His reforms sought a stronger, more service-oriented state. He established the Senate, reorganized central administration into colleges, promoted technical education and encouraged foreign expertise. The Table of Ranks, introduced in 1722, linked official standing to military and civil service, while still operating within a deeply unequal society. A poll tax supported the expanding army and administration. He also reshaped elite dress and court customs. After the patriarchate's vacancy, he placed church administration under a Holy Synod in 1721, strengthening the monarchy's control over religious institutions.

The reforms depended on coercion as well as innovation. Taxation, conscription and forced labour imposed severe burdens, on peasants; resistance met punishment. Peter suppressed the streltsy revolt of 1698 and later prosecuted his son Alexei, who died in custody in 1718. His final campaigns added territories along the Caspian, although these gains proved less durable than the Baltic settlement. At his death in 1725 he left Russia with enlarged military power, a new capital and altered governing institutions, together with the social costs of an intensely autocratic transformation.