Liberalism is the ideology of capitalism. They emerged together and the former was formed to justify the latter. Over the years it has branched out and there are many forms such as classical liberalism, neoliberalism, social liberalism, etc. but they all defend capitalist property rights and the market. Socialism emerged as the working class response to/critique of liberalism. In the US the term only refers to social liberals, who are in reality centrists. Americans call them leftists only because centrists are slightly to the left of right-wing politics.

We're against liberalism as a whole because it's the ideology that justifies capitalism. We're against social liberals because they're seen as fence-sitting cowards and dangerous compromisers.

This is a very introductory overview to liberalism:

• Taimur Rahman - Introduction to classical liberalism - John Locke

The most in-depth delving into it is Losurdo's Liberalism - A counter history, but you'd have to read many more foundational texts before that one.