Simply expressed they were a consortium of American misfits, anarchists, communists, intellectual half-wits, socialists, and thugs who ventured to Spain in the 1930s to ostensibly save Spain from Republicanism and Fascism. The truth beneath the boil is the ABL went there with the help of the Soviet Union to establish a Communist state. This adventure coined the phrase "anti-fascist". If you weren't with them then you were a fascist or right-wing extremist and corporatist. Wisely then, unlike our warmongering today, the US had a neutrality doctrine which kept our government rightfully uninvolved.
The ABL has many of the character traits of what's building again in our homeland. Our colleges, under the prodding and urging of the many Marxist professors, can quickly spill thousands of unemployed 'student' "trabajadores" onto our streets and in our face. Note "Lincoln" in the banner to the right as a component of the Internacional Brigades.
Contemporary socialists, progressives, Marxists, and communists keep the ABL alive agitating for an anarchist world from which can rise a Communist utopia.