The antagonist of my story is planning to prosecute a war on as many as three fronts. What historical examples could I draw on to make such an operation seem plausible, even if it's doomed to failure in the end? Are there any best practices that a competent (if insane) antagonist should employ in this situation?
Technology in this world doesn't neatly map to a particular era of real-world history, but is entirely pre-modern (nothing past the early renaissance era).
My antagonist has recently invented firearms/explosives and rudimentary chemical weapons, and has equipped his army with them. No one else has this technology.
The antagonist is fighting nations that have access to powerful magic and is relying on his firearms and chemical weapons to counter this advantage. Detailing the magic he's up against would probably expand the scope of this question too much, but more info will be provided if requested.
This takes place across subtropical/tropical regions. My antagonist's army will have to contend with savanna, subtropical forest, tropical rainforest, and potentially swamp and desert environments. His enemies will obviously have the home field advantage, which I expect will be especially significant in jungle, swamp, and desert.
Two of the nations my antagonist is invading have long coastlines, and one has an abundance of navigable rivers. My antagonist's naval strength is meh but he has strong naval allies. Only one of the nations he is fighting has any navy to speak of.
My antagonist's soldiers have no magic whatsoever, but the antagonist himself is a powerful and intelligent warlock. He is supplementing his human forces with chimeric monsters, which will make up the bulk of his jungle invasion force and provide support as needed elsewhere.
As his trump card, my antagonist intends to deploy a reanimated dragon, though this will take time to prepare and requires access to key resources currently in enemy territory. Due to both lack of knowledge and cultural taboos, there is zero danger of his enemies pulling an uno-reverse and using a reanimated dragon against him. Dragons have been extinct for millennia and the tactics previously used to extinct them are all but lost.