Toward the end of the First World War, the Central and Eastern Europe knew a wave of national claims, in the sense of the right for the “self-determination of the peoples”, which was solemnly acknowledged by the President Woodrow Wilson, and ideologically declared by Lenin. In the case of the Romanians, the way toward the Great Union was marked by diplomatic conventions, as those with the Entente’s Powers, in 1916, and by the belligerent effort during the years 1916-1918. In the Union’s Acts, as the claims of the Romanians in Bessarabia, Bucovina and Transylvania, reverberated the national principle, as it was consecrated by the French Revolution and was confirmed by the new historical course of Europe. Some reflections on that principle should be useful for the scholarly debates concerning the legitimacy of the Romanians Great Union of 1918.