Celebrating Tarzan's 101st anniversary by walking through Scott Tracy Griffin's Tarzan: The Centennial Celebration.
Two other Tarzan novels were published after Tarzan and the Foreign Legion, but this was the last one that Burroughs wrote. It was also the only one he wrote after the Japanese attack on Pearl Harbor in 1941. Burroughs was living in Hawaii at the time and actually witnessed the bombing. His six-year marriage to young Florence Dearholt had ended earlier that year and he was already deeply depressed, but he channelled those emotions into writing morale columns for a couple of Honolulu newspapers. A year later, he was attached as a war correspondent with the U.S. Army in the South Pacific.
When he finished that tour in 1944, Burroughs got a letter suggesting he write a Tarzan story in which the ape man fought the Japanese. It had never worked out great when Burroughs had included real-world villains in his novels before, so he resisted at first, but eventually decided to do it. It was Burroughs' first
Tarzan doesn't join the actual French Foreign Legion in the book, but enlists in the RAF and gets attached to the U.S. Army Air Force in a recon mission over Sumatra. When the plane is shot down, Tarzan leads his diverse unit (nicknamed "The Foreign Legion" by one of its members) through the Japanese-occupied jungle in an attempt to reach the coast, build a raft, and sail for Australia. They of course have to fight Asian jungle wildlife along the way and end up discovering a lost race of pygmies.
Griffin's supplemental chapter for this one is called "Implacable Foes" and details the various types of villains Tarzan encounters in his adventures from sentient animals to spies to slavers to holy men to treasure hunters. Griffin also lists some of the great actors who played Tarzan bad guys in the movies, including Sean Connery, John Carradine, Raymond Burr, Boris Karloff, Jack Elam, George Zucco, and a few fellas who would go on to play the ape man himself. (Sadly, he doesn't mention one of my favorites: Neil Hamilton, who's best known as Commissioner Gordon on the '60s Batman TV show.)