It, and other London works, promoted individualism and attacked totalitarianism.
Let's go back to the plot of The Call of the Wild. Rereading it, there are a few main incidents that jump out at me as part of a larger trend:
- Buck's attack in Seattle on the "stout man, with a red sweater" who threatens him with a hatchet and club.
- His rivalry with Spitz for the lead of the sled dog team and his eventual victory and deadly overthrow of the lead dog.
- His attack on the Yeehats in revenge for their murder of John Thornton.
- His eventual life with the wolves as a free dog.
There's a common pattern here: fighting back. For a sled dog, life in the Yukon means fighting for your position and sometimes your life, as per The Law of Club and Fang. Additionally, abuse by masters requires self-defense. Buck quickly learns that he needs to protect himself if he wants to survive. In a way, Buck symbolizes the author himself. In his essay, "How I Became a Socialist", London wrote
I was a rampant individualist. It was very natural. I was a winner.
Individualism is terrifying for those in power. Individualism leads to fighting back against injustice, and that in turn can lead to revolution. In the 1920s - two decades after The Call of the Wild was published - Italy and Yugoslavia were ruled by dictators, Benito Mussolini and Alexander I, respectively. For both, rebellions would serve as their downfall later in their reigns, after various periods of time. The Call of the Wild could inspire revolution, and so was dismissed as "too radical". Banning it was a natural extension.
The ban on The Call of the Wild was not an isolated incident. Since his rise to power, Mussolini took decisive action against writings of all kind. As Guido Bonsaver's Censorship and Literature in Fascist Italy explains, the dictator first targeted journalists, newspapers and periodicals in the 1920s. He only engaged in large-scale book-banning campaigns starting around 1931. Now, lest you think that the characterization of The Call of the Wild as too radical is preemptive, let me note that London's The Iron Heel (as well as various works by Maxim Gorky) were gathered and favored by anti-Fascists in Italy (p. 37). In other words, Mussolini had good reason to believe that London's works were actually inspiring revolution.
I'm less familiar with The Iron Heel, but I'll take a shot at analyzing it. It focuses on a period stretching from 1912 to 1932 - really, leading up to and encompassing Mussolini's rise to power and early years. Socialism managed to become popular on both the United States and Europe, eventually leading to essentially the overthrow of governments. Once again, we see a popular uprising attacking the status quo, be it fascism or otherwise. This would be rightly worrisome for Mussolini and, later, Hitler.
The Iron Heel and The Call of the Wild were not the only of London' works to essentially oppose the Nazi regime, even after his death in 1916. The Sea-Wolf, published in 1904, was written - according to London in The Letters of Jack London (p. 1513) - to attack the Übermensch ("super-man") philosophy espoused by, among others, Friedrich Nietzsche. (London's Martin Eden also was strongly critical of the same ideas.)
In the case of The Sea-Wolf, the embodiment of Nietzsche's super-man is Wolf Larsen, the captain of a schooner named the Ghost. Larsen is
- Extremely smart.
- Strong, and not afraid to use intimidation.
We see plenty of parallels between the life of the protagonist (Humphrey van Weyden) on the Ghost and Buck's struggle for survival in The Call of the Wild. It's an environment not suited for the weak or meek, where both the captain and crew can turn against you. While van Weyden is favored by Larsen, especially after not taking part in a mutiny, he is clearly a rival of the captain, just as Buck was a rival of Spitz.
The difference here is that Larsen is truly a super-man - and he is portrayed as cruel and cold. Essentially, the book would later be used to attack the Nazis' master race doctrine. Add on the indirect promotion of individualism and socialism, and London becomes a cocktail for revolution against the dictatorships of Europe.