The important point is that liberalism emerged as part of the same historical moment as the development of capitalism, the rise of European colonialism, the Atlantic slave trade and race 'science'. Liberalism was always implicated in these processes, from Locke to Tocqueville. US liberals of the Progressive era were generally explicit imperialists and white supremacists, none less than the paladin of 'liberal internationalism', Woodrow Wilson. What you refer to as the "bombing Left" is part of the same history. Large parts of the developing left and labour movements in the 19th Century partook of the colonial triumphalism and associated doctrines such as 'social Darwinism'. The regnant view was that much of the human race was bound for extinction if it wasn't possible to civilize them. Thus Karl Kautsky argued in 1882 that: "In so far as they cannot be assimilated by modern culture, the wild peoples will have to disappear from the surface of the earth."
(...) Left-wing apologists for imperialism also borrowed a chauvinistic version of humanitarianism from the liberals, namely the idea that - as Eduard Bernstein put it - the "savages" under colonial rule were "without exception better off than they were before". The Fabians similarly believed that self-rule was as useless to non-white people as "a dynamo to a Caribbean" and that for their own benefit it was necessary to impose the "grandmotherly tyranny" of colonialism. Labour's 1919 manifesto, radical in so many other ways, continued to enjoin Britain's duty to the "non-adult races". The Russian Revolution, and ensuing national liberation struggles often fought under the impress of some kind of marxism, put those explicitly advocating empire on the back foot: they had to change the terms of their argument, and they did so with reference to the exigencies of containing communism. Even then, the racial and colonial aspects of American dominance took a while to be suppressed. The USSR was itself considered an "oriental despotism" resulting, according to George F Kennan, from "a century-long contact with the Asiatic hordes" whose effects had only been concealed by the "Westernised upper crust of the Tsarist elite". Of especial concern was the commie attempt to weaken the power of Western states in colonised nations, which produced a fear of "premature independence" for those not yet adequately schooled in the arts of government by whitey, who might prove easy meat for the Muscovite menace.