Prince Ferdinand ascended the throne of Romania on October 11, 1914, after the death of King Carol I. It was a crucial period in the history of Romania, marked by the outbreak of World War I, in which both the royal family and the entire Romanian society were deeply divided into camps. either neutrality or going to war on the side of one or the other of the two conflicting alliances. By the death of King Carol I, a tense political situation had unraveled. In the parliament, the new leading pair was received with warmth and hope, being long acclaimed.
After the serial defeats suffered by the Romanian Army in Oltenia and Muntenia, the loss of the battle for Bucharest definitively compromised the situation of the head of the Army's General Headquarters, Brigadier General Dumitru Iliescu. Ferdinand resolutely intervenes and asks the prime minister to remove his protégé and to leave the current leadership of the operations of the army of the General Headquarters, without the interference of the Ministry of War he was leading.
After the signing of the Armistice in Focșani with the Central Powers on November 26, 1917, the relations between Queen Maria on the one hand, King Ferdinand, Ion I.C. Brătianu and Barbu Știrbey are deteriorating, as a result of their position on divergent positions regarding the future action. Maria considers the armistice a gear in which Romania got its hands on it, while Brătianu and Știrbey consider it a diplomatic maneuver meant to gain time. Future developments will confirm the queen's point of view, since then the Central Powers have done nothing but "tighten the gear screw" leading in just three months to the signing of the humiliating separate peace. The inability of the country's political leadership to identify a viable solution, as well as the unfavorable external context, forced King Ferdinand to accept a meeting with the Minister of the Austro-Hungarian Empire, Count Czernin, on February 27, 1918, who in an arrogant and humiliating tone , asks him to sign the peace or he will be replaced by another king from the Austrian or German royal houses. "The king cried and gave the impression that he wanted to make peace but that he was still in the hands of those around him," records Alexandru Marghiloman.
Eventually, the situation would change. During 1918, the year of the birth of Greater Romania, the situation of the war turned against the Central Powers and Ferdinand returned to Bucharest at the head of the army, passing under the Arc de Triomphe, being greeted by the enthusiastic population. The Romanian army reached Budapest, entering the Hungarian capital on August 4, 1919, and liberating Hungary from the communist regime of Béla Kun. The latter fled, via Vienna, to the USSR.
Ferdinand was crowned King of Greater Romania on October 15, 1922 in the Cathedral of Alba Iulia. Domestic political life during his reign was dominated by the National Liberal Party, led by brothers Ion Brătianu and Vintilă Brătianu. However, the union with Transylvania widened the electoral base of the opposition, whose main parties united in January 1925 - October 1926 to form the National Peasant Party.