Second, the threat of fascism was far more than merely political. What was at issue - and nobody was more aware of this than intellectuals - was the future of an entire civilisation. If fascism stamped out Marx, it equally stamped out Voltaire and John Stuart Mill. It rejected liberalism in all its forms as implacably as socialism and communism. It rejected the entire heritage of the eighteenth-century Enlightenment together with all regimes sprung from the American and the French Revolutions along with the Russian Revolution. Communists and liberals, confronted by the same enemy and the same threat of annihilation were inevitably pressed into the same camp. It is impossible to understand the reluctance of men and women on the left to criticise, or even often to admit to themselves, what was happening in the USSR in those years, or the isolation of the USSR's critics on the left, without this sense that in the fight against fascism, communism and liberalism were, in profound senses, fighting for the same cause. Not to mention the more obvious fact that each needed the other and that, in the conditions of the 1930s, what Stalin did was a Russian problem, however shocking, whereas what Hitler did was a threat everywhere. This threat was immediately dramatised by the abolition of constitutional and democratic government, the concentration camps, the burnings of books, and the massive expulsion or emigration of political dissidents and Jews, including the flower of German intellectual life. What the history of Italian fascism had hitherto only hinted at now became explicit and visible to even the most short-sighted.
~~ Eric Hobsbawm, How To Change The World: Marx and Marxism 1840-2011 (p. 268)
There are very few historical instances in which liberals (or what we would now call progressives in current US parlance) and leftists have found common ground. In the case discussed above, Hobsbawm starts by discussing why liberal and Marxist intellectuals lent support - often tangible - to the defenders of Spain's Republic and against the Falangists during the Spanish Civil War, and places that in the context of the rather violent rise of fascist regimes on the European continent.