Affected by heart disease from childhood, Diamandy had to maintain a low profile in politics, but was a vocal marginal within the National Liberal establishment. From 1910, he invested his energy in literature and cultural activism, chairing the National Theater Bucharest and later the Romanian Writers' Society. He was pushed back to the forefront during the early stages World War I, when he supported an alliance with the Entente Powers. He advised Premier Ion I. C. Brătianu on the matter and was sent on diplomatic missions to the West, helping to cement France's trust for Romania. He fought in the ill-fated campaign of 1916, and withdrew to Iași, retaking his seat in Chamber.