In World War I, Italy completed the unification by acquiring Trento and Trieste, and gained a permanent seat in the League of Nations's executive council. Nevertheless, Italian nationalists considered World War I a mutilated victory because Italy did not have all the territories promised by the Treaty of London (1915) and that sentiment led to the rise of the fascist dictatorship of Benito Mussolini in 1922. The subsequent participation in World War II with the Axis powers, together with Nazi Germany and the Empire of Japan, ended in military defeat, Mussolini's arrest and escape (aided by the German dictator Adolf Hitler), and an Italian Civil War between the Italian Resistance (aided by the Kingdom, now a co-belligerent of the Allies) and a nazifascist puppet state known as "the Italian Social Republic". Following the liberation of Italy, the fall of the Social Republic and the killing of Mussolini at the hands of the Resistance, the country abolished the monarchy with a referendum, reinstated democracy, enjoyed an economic miracle, and founded the European Union, NATO, and the Group of Six (later G7 and G20). It remains a strong economic, cultural, military and political factor in the 21st century.