Okay, so there is no way to go about reviewing Behemoth if nothing is said about Leviathan, even if both books stand on their own well enough. Except if you read Behemoth first, you’d want to go right back out and get Leviathan anyway, to make sure you got the full experience.
Leviathan is set at the beginning of World War I, with the death of Archduke Ferdinand by Serbs. As such, we can’t exactly pin it down to the era of steam technology, so it’s more fittingly dieselpunk. Nonetheless, the historicity and scale of tech retrofitted into the past fit nicely into steampunk conventions.
Within this history, it’s obvious that Westerfeld has done his homework, down to little details that add a delicious accuracy to enhance certain scenes, while being very clear where he has strayed. As such, there isn’t one break-off point between this story and recorded history, but a blend of both.
The two major factions within the new geopolitical landscape are very reasonably set: in the bits of Europe that is Catholic, the predominant tech is mechanical, with hulking machines that are deeply reminiscent of HG Wells’ land ironclads. The British, by contrast, are Darwinists, with the conceit that Darwin discovered DNA and developed the technology to harness it, to the point that the British fabricate their own biological ecosystems in a fashion that serves their purposes.
This is how we get Leviathan, which is, to put it bluntly, a flying whale.