I'm not sure it's all that useful. To me it seems more like an argument that some forms of tyranny need to be continued because there's an outside threat. And there were people who knew better about the Jews in the 1930s. I don't claim to understand anti-semitism, but I think it's a confluence of misunderstanding Christianity's origins in Judaism, raw xenophobia, and simple scapegoatism. It never had any merit. FDR should never have turned away the St. Louis, and it's an embarassing mark on our history that he did.
By the way, the Japanese were preaching anti-semitism (i.e. blaming "the international Jew") before and during WWII, as well.
In any case, the Belloc clearly doesn't get why the west has been so successful: it's because we've abandoned irrationalism. He blames our supposedly impending downfall (in 1939 this would have been easy) on the abandonment of tradition. This is a very anti-American, anti-modern, and superstitious. How familiar, as we hear that sort of thing today.
The west will survive the current Islamic threat without this guy's alarmist advice.