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In current use, the noun Antifa (also with lower-case initial) refers to a political protest movement comprising autonomous groups affiliated by their militant opposition to fascism and other forms of extreme right-wing ideology.
This noun originally referred to anti-fascist committees in Germany at the end of, and immediately after, the Second World War (1939-45).
According to the Oxford English Dictionary (current online edition), in that original sense, this noun is a borrowing from German Antifa, shortened from the noun Antifaschismus (i.e., anti-fascism) and from the adjective antifaschistisch (i.e., anti-fascist) as in antifaschistische Bewegung (i.e., anti-fascist movement).
These are, in chronological order, the earliest occurrences of the noun Antifa that I have found in texts written in English:
1-: From Germany Gets Back to Politics, by J. F. Lorenz, published in the Manchester Evening News (Manchester, Lancashire, England) of Wednesday 26th September 1945 [page 2, column 3]:
During the first days of the occupation Communists and Social Democrats have very often set up local committees under the name of Antifa, which is short for Anti-Fascist Group. Whether these Antifa will be the forerunner of a united Popular Front remains to be seen. In some places the Antifa have already split again into their component parts, in others it has been decided to keep the Antifa in being and to fight the first local elections on a Popular Front basis.
2-: From 70,000 Trade Unionists in Hamburg Co-operating with Military Government, a correspondence from Hamburg, Germany, dated Thursday 11th October 1945, published in The Manchester Guardian (Manchester, Lancashire, England) of Friday 12th October 1945 [page 8, column 6]:
Membership of the Communist party, from which the Socialists hold themselves completely aloof, is about fifteen thousand in Hamburg. Other branches are in Kiel, Lübeck, Eckernforde, and Flensburg. The party claims to be independent of both Moscow and Berlin. In addition to the political parties there are numerous small anti-Fascist groups organised on Popular Front lines, notably Antifa in Lübeck, and Antinazi Aktion in Neumünster. These two bodies have been particularly helpful in regard to de-nazification.
3-: From Germany in Defeat, by ‘G. K. Y’, published in The World Today (London: Royal Institute of International Affairs) of February 1946 [page 73]:
Between April and July, 1945 the most widely-observed political phenomenon was the appearance of local anti-Fascist committees, usually named “Antifa” for short. Some had existed in embryonic form since the beginning of the year, when growing confusion had weakened Gestapo control. Communists played an important part in their formation, not as a result of any national or international direction, but because any survivors of underground Communist cells knew the technique of the clandestine meeting and the passing of notes. They were sometimes joined by foreign workers of Left-Wing sympathy. As part of its “no politics” directive, Military Government did not encourage such committees: only occasionally were they used in an advisory capacity. Generally the “Antifa” was passed to Field Security, who were glad to use its information about local Nazi bigwigs. “Antifas” have gradually languished, particularly as a strong revulsion against denouncing and informing has set in—in itself a significant point.
It is a matter of speculation whether the course of German politics would have been different if “Antifa” had been encouraged. Local anti-Fascists do not see the point of a “no politics” rule which bans them but leaves Fascist officials in office. Even when this situation is later remedied by Field Security arresting the officials in question, a feeling of disillusionment and scepticism over Allied intentions remains. This is a factor of some importance in the Left-Wing attitude towards British occupation authorities. Some “Antifa” groups remain in Westphalia and Hanover and their members will probably participate in any local united fronts formed by Socialists and Communists.
4-: From The Problem Of Germany, an editorial published in The Catholic Weekly (Sydney, New South Wales, Australia) of Thursday 14th February 1946 [page 4, column 1]:
In the French Zone all political parties are banned, although an anti-Fascist organisation called “Anti-Fa” is permitted.
5-: From a review of two books (Experiment in Germany: The Story of an American Intelligence Officer, by Saul K. Padover, and America’s Germany: An Account of the Occupation, by Julian Bach Jr.)—review by the U.S. Neo-Marxist economist and political activist Paul Sweezy (1910-2004), published in The New Republic: A Journal of Opinion (New York City, New York, USA) of Monday 22nd April 1946 [pages 585 to 588]:
[page 587, column 1]:
The American public knows nothing about the proto-revolutionary movement which sprang up in city after city as the Allied armies swept eastward from the Rhine. Organizationally, this movement took the form of anti-fascist committees led in most cases by Communists or left-wing Social Democrats. These committees had a variety of local titles (e.g., in Bremen Kampfgemeinschaft gegen den Faschismus, in Leipzig Nationalkomitee Freies Deutschland, etc.), but they soon became widely known under the generic name of Antifa (for anti-fascist). In many cases they undertook direct action against Nazis, resisting notorious local bosses, confiscating food hoards and evicting party members from their houses in favor of the bombed-out.
[page 588, column 1]—MG stands for Military Government:
But the Antifa movement never got a chance to develop. Direct action against Nazis was, in the view of the MG, “illegal” and subversive of law and order. Buildings which had been seized for headquarters (in Bremen the building which had belonged to the Labor Front) were taken over by the MG. In many cases the organizations were suppressed out of hand; American (and British) officers weren’t going to have anyone telling them how to run their business; and besides, how could you be sure that these weren’t Nazi outfits in disguise? The result was that within a month or so most of the Antifas, deprived of the possibility of functioning openly and being in principle opposed to the formation of an underground movement against the Allies, dropped out of the picture. When political activity was once again permitted, it ran for the most part into the old party channels. It is this reviewer’s considered judgment that the Antifas represented the beginnings of a genuine revival of working-class revolutionary activity, and that their strangulation gave the played-out politicos of the Weimar period a new opportunity to divide the workers and to reinstate themselves in party and trade-union bureaucracies.