Peter I (1844-1921) was king of Serbia from 1903 to 1918 and of the Serbs, Croats, and Slovenes from 1918 to 1921. He introduced constitutional monarchy to Serbia and was the first post-World War I monarch of the unified Yugoslav state.

Peter was born in Belgrade on July 11, 1844. His father was Prince Alexander Karageorgevic. After his older brother died, Peter became heir to the Serbian principality, which his father had ruled since 1842. However, in 1858, while Peter was abroad in Switzerland, his father was forced to abdicate in favor of the rival Obrenovic family. The young prince then began a long period of exile abroad in the service of other states.

After completing his studies in France, Peter entered the French officer corps in 1867. He fought in the Franco-Prussian War of 1871. Returning to southern Europe, in 1875 he joined the Herzegovinian revolt against Turkey, which soon spread to the South Slav territories still under direct Turkish control. Austrian intervention in 1876, however, forced Peter to leave the area.

In 1883 Peter married Zorka, daughter of Prince Nicholas of Montenegro, in which state he also became an honorary senator. After his wife's death in 1890 he returned to Switzerland.

In 1903 King Alexander of Serbia was assassinated, and Peter returned to his homeland as constitutional monarch. Peter I was crowned in Belgrade on Sept. 21, 1904. The aging Peter pursued liberal policies, but by the time Serbia was engulfed by World War I, he had already appointed his heir, Alexander, as regent.

Peter's vitality returned when war broke out. Although he was forced to retreat across Albania late in 1915, the King became the focus of national resistance. At the war's end Peter returned to his capital; there he was proclaimed king of a new South Slav state on Dec. 1, 1918.

Unfortunately, the new state over which Peter I ruled was large enough to threaten Italian interests. The last years of his reign were marked by a contest with Italy over the town of Fiume. Negotiations between the two countries saw the Yugoslavs forced to grant Fiume status as a free port. When Peter died near Belgrade on Aug. 16, 1921, the issue was still feeding fuel to the fires of Italian nationalism.