Basically, the world follows the beginning of World War I, but instead of just Germans versus allies, the sides are broken up more clearly as Clankers vs Darwinists, respectively. The countries on the Clankers side have their technology based in diesel machines, often with legs (the typical steampunk aesthetic), while the Darwinists have genetically modified animals that are used as weaponry and technology, such as the titular giant whale that is part war machine and modified to be have a hydrogen bladder, so it can fly like a blimp.
Puts more of a versus thing onto world war i, since that war was a lot less cut in stone. It was set based more on interlocking ally-ships causing a war after the archduke was assassinated.
Beyond that, the story follows the timeline of World War I, with the exception that Archduke Franz Ferdinand's nephew (now the heir to the throne) survived and is in hiding, and is one of the main characters.
Puts more of a versus thing onto world war i, since that war was a lot less cut in stone. It was set based more on interlocking ally-ships causing a war after the archduke was assassinated.
Having a much more clear cut us vs. them conflict for WWI is an odd choice, seeing as how the real war was mainly a series of unfortunate events (starting with the Archduke's assassination) paired with a lot of ally-ships causing tensions to rise and explode in one big war. It was more a war with sides but no stances, unlike WWII. 'Leviathan' takes this war and puts more solid ethical and technological differences between the two sides. I guess the reasoning Westerfeld added this element in was to make the moral of the book (that uniting and compromise are important) more clear-cut for younger audiences, but I feel he may have chosen an odd war to do this with. But I do supposed it would have been harder to push that message with any war involving Nazis.
Honestly, this novel didn't seem to have a good reason besides cool worldbuilding and aesthetic purposes to add the steampunk elements, and I wouldn't say it focused super hard on the moral implications of either of the sides like some other cyber/steampunk stories do. While something like 'Blade Runner' uses the androids to question humanity and what makes us human, 'Leviathan' barely grazes the moral implications of genetically modifying animals to be mashed with war machines. Perhaps the rest of the trilogy goes more into this, but the first book mainly sets up the world, the conflict, and the characters.
That being said, the morality of the Darwinists movement is something the reader themselves can ponder on, which perhaps was Westerfeld's intention (I have read another one of his series before, the 'Uglies', and he did focus heavily on the impact of technology on society, so I have hope this was intentional). I do think though that the steampunk elements were a bit tone deaf for the time period Westerfeld chose.
It was a fun read though, and the art was beautiful.