Karl of Hohenzollern-Sigmaringen arrived in 1866 by riverboat under a false name, traveling through Austrian territory to evade detection. He was twenty-seven, a Prussian officer, and almost completely unknown to the people he was about to rule. He learned Romanian, Romanianized the spelling of his name to Carol, married Princess Elisabeth of Wied, and set about building. During his reign Romania got its first university expansion, its first national bank in 1880, the Cernavodă Bridge (then the longest in Europe), the Peleș Castle in Sinaia, and a railway network that grew from a few hundred kilometers to thousands. The 1877-78 Russo-Turkish War, in which Romanian troops fought alongside the Russians at Plevna, secured the kingdom's independence from the Ottomans. By the time Carol died in October 1914, Romania was a recognizable modern European state.

The First World War transformed everything. Romania entered the war in 1916 on the Allied side and was immediately overrun by the Central Powers, losing Bucharest and the oil fields by year's end. The army held a desperate line at Mărășești in 1917 but had to sign an armistice when Russia collapsed. Then everything reversed. The Central Powers fell in November 1918, and at the peace conferences Romania emerged with Transylvania, Bessarabia, Bukovina, and parts of the Banat: more than double its prewar territory. King Ferdinand I, who had succeeded his uncle Carol, was crowned King of all the Romanians at Alba Iulia in 1922. The new state was a multinational one, with significant Hungarian, German, Jewish, and Ukrainian minorities whose rights were unevenly protected. The interbellum years brought an oil boom (Romania was a top global oil exporter by the late 1930s) and political instability that would prove fatal.

Ferdinand died in 1927. His son Carol had renounced his rights to the throne over a scandalous affair with Magda Lupescu, a divorced Romanian woman of partly Jewish ancestry, and lived in exile while his young son Michael was crowned. In 1930 Carol changed his mind, returned, took the crown for himself as Carol II, and restored Lupescu to his side, breaking promises he had made to politicians who supported his return. The 1930s in Romania were a dark decade. Over twenty-five governments came and went. The fascist Iron Guard, founded by Corneliu Zelea Codreanu, used assassination as policy, killing Prime Minister Ion Duca in 1933 and Prime Minister Armand Călinescu in 1939. Codreanu and other Iron Guard leaders were themselves killed in November 1938 in what was officially called an escape attempt and almost universally believed to be murder by the king's order. In February 1938, Carol II suspended the constitution and declared a royal dictatorship.

In the summer of 1940, under Soviet, German, Hungarian, and Bulgarian pressure, Romania lost Bessarabia, northern Bukovina, northern Transylvania, and southern Dobruja. Carol II abdicated in September and fled the country with Lupescu. His teenage son Michael returned to the throne, but real power belonged to General Ion Antonescu, who governed as Conducător in alliance with Nazi Germany. Romanian troops fought alongside the Wehrmacht at Stalingrad. Romania participated in the Holocaust on its own territory and in occupied Soviet lands; estimates of Romanian Jews and Roma killed under Antonescu's regime range from approximately 280,000 to 380,000, with thousands more deported. On August 23, 1944, with Soviet armies advancing, the twenty-two-year-old King Michael led a coup, arrested Antonescu, and switched Romania to the Allied side. The coup is widely credited with shortening the war by months.

The cost of the coup was high. Soviet troops occupied the country, and through the next three years the Romanian Communist Party, working with Soviet backing, took over the institutions of state. By 1947 Michael was a king without power. On December 30, with Communist leaders in his palace and troops outside, he was forced to sign an act of abdication for himself and his descendants. He was twenty-six. He left Romania the following month and lived most of the rest of his life in exile, much of it in Switzerland, working as a test pilot and a stockbroker. He visited Romania for the first time after the fall of communism in 1992 and was greeted by enormous crowds. He died in December 2017, at ninety-six, the last surviving head of state of the Second World War. The Kingdom he had inherited at five years old, lost at nine, regained at nineteen, and surrendered at twenty-six was gone for good, but its memory remained more complicated than the official histories had ever allowed.