With the development and spread of Italian Fascism, i.e. original fascism, the National Fascist Party's ideology was met with increasingly militant opposition by Italian communists and socialists. Organizations such as the Arditi del Popolo and the Italian Anarchist Union emerged between 1919–1921, to combat the nationalist and fascist surge of the post-World War I period. Thus, as fascism coalesced into a coherent ideology, a militant leftist opposition sprouted in response.
In the words of historian Eric Hobsbawm, as fascism developed and spread, a "nationalism of the left" developed in those nations threatened by Italian irredentism (e.g. in the Balkans, and Albania in particular). After the outbreak of World War II, the Albanian, and Serbian resistances were instrumental in antifascist action and underground resistance. This combination of irreconcilable nationalisms and leftist partisans constitute the earliest roots of European anti-fascism. Less militant forms of anti-fascism arose later. For instance, during the 1930s in Britain, "Christians – especially the Church of England – provided both a language of opposition to fascism and inspired anti-fascist action".
The diversity of political entities that share only their anti-fascism has prompted the historian Norman Davies to argue in his book Europe at War 1939–1945: No Simple Victory that anti-fascism does not offer a coherent political ideology, but rather that it is an "empty vessel". Davies further asserts that the concept of anti-fascism is a "mere political dance" created by Josef Stalin and spread by Soviet propaganda organs in an attempt to create the false impression that Western democrats by joining the USSR in the opposition to fascism could in general align themselves politically with communism. The motive would be to lend legitimacy to the dictatorship of the proletariat and was done at the time the USSR was pursuing a policy of collective security. Davies goes on to point out that with Winston Churchill as a notable exception, the concept of anti-fascism gained widespread support in the West, except that its credibility suffered a serious but temporary blow while the USSR and Nazi Germany coordinated their wars of aggression in Eastern Europe under their Molotov–Ribbentrop Pact.
Anti-fascist movements emerged first in Italy, during the rise of Mussolini, but soon spread to other European countries and then globally. In the early period, Communist, socialist, anarchist and Christian workers and intellectuals were involved. Until 1928, the period of the United front, there was significant collaboration between the Communists and non-Communist anti-fascists. In 1928, the Comintern instituted its ultra-left "Third Period" policies, ending co-operation with other left groups, and denouncing social democrats as "social fascists". From 1934 until the Molotov-Ribbentrop Pact, the Communists pursued a Popular Front approach, of building broad-based coalitions with liberal and even conservative anti-fascists. As fascism consolidated its power, and especially during World War II, anti-fascism largely took the form of Partisan or Resistance movements.
In Italy, Benito Mussolini's Fascist regime used the term "anti-fascist" to describe its opponents. Mussolini's secret police was officially known as Organizzazione per la Vigilanza e la Repressione dell'Antifascismo (OVRA), Italian for "Organization for Vigilance and Repression of Anti-Fascism").