An Anglophile promoting an alliance with the Triple Entente, he rallied politicians and intellectuals in support for the idea of Romania entering World War I. When this was accomplished through the 1916–1918 campaign, Ionescu joined the Ion I. C. Brătianu government in Iași as Minister without portfolio. After his country was defeated by the Central Powers and signed the Treaty of Bucharest, he left for Paris and London, organizing a Romanian National Committee to campaign for Greater Romania during the Peace Conference. In 1919, his Conservative-Democrats formed an alliance with the People's League, and Ionescu became Foreign Affairs Minister in the second Alexandru Averescu executive, before briefly holding the office of Premier in 1921–1922. During the period, he successfully campaigned in favor of the Little Entente.