It's not that liberal democracies always slide, specifically, it's that Capitalist states always slide, and this is heightened by being in the Global North. Global North countries brutally explioit Global South countries via Imperialism, by relying on vastly under-paid labor and selling it in the Global North for higher prices.

Fascism is Capitalism in decay, the violent immune system employed by the Capitalist class. A great work on fascism is Blackshirts and Reds. I can provide a longer Marxism intro reading list if you'd like, but Blackshirts is a great start.

I also recommend Imperialism, the Highest Stage of Capitalism, and the famous Yellow Parenti Speech (a small excerpt here.