I jumped on the Pocket Guides for a quick peek and I found some interesting quotes pertaining to the questions.
But is there a real recorded history for this sort of war mongering behavior?
"There is no standing army in Elsweyr, and the catmen have never shown an expansionist inclination. In fact, they have lost territory in the last fifty years with the secession of their rim territories "
"Elsweyr has withdrew itself into a secrecy that has scarcely been breached in five hundred years."
It seems that Elsweyr isnt aggressive and is simply self absorbed.
I know that they aren't fond of Bosmer under the merit that they sort of consider them their black sheep cousins.
"They have engaged in wars with Valenwood on grounds that have constantly shifted, like the sands of Elsweyr itself."
It was said in one of the in-game books or a rumor in Oblivion or somewhere that the Khajiits attacked because the Bosmer wiped out one of their villages in response to cutting down trees (possibly to hold back the forest growth?). Theres no hard evidence so its most likely just typical border raids that piss off one side or the other and starts a conflict. I remember a khajiit saying that there is always some sort of fighting going on between Vallenwood and Elswyre.
I can also understand why they wouldn't like the empire, because the empire is in power and nobody likes the top dog.
"The old kingdom of Anequina is its northern section, and has historically offered no threat to either the early Cyro-Nords or the later Cyrodiliic Empires"
"Human relations have been minimal in the intervening years, but there is talk that the Elsweyr Confedracy has recently struck treaty with the Aldmeri Dominion, a situation that, if true, may force the catmen into another bloody confrontation with the Cyrodilic masters of Tamriel."
"The Rimmen (literally, the "Rim Men," as the khajiit called them) joined their brothers to try to rebuild the Empire. This effort was doomed to failure, but not before the khajiit attempted to reclaim their lands in a series of bloody border wars."
"The Khajiit have found security in being absorbed into the Cyrodilic and then the Septim Empires, only to rebel against both"
It seems, from these snippets, that the Khajiit are simply opportunists that jump at any chance to reclaim their land no matter who they fight but aren't particularly hungry for more territory.
Is it just that Khajiits are naturally antagonistic? They ARE a nomadic people who even fight amongst themselves quite a bit, so they're not strangers to conflict.
"Chieftains forgot their ancient sugar-vendettas and signed treaties of their own"
"On the whole, the catmen of Elsweyr are a bestial lot, victims of their own preternatural anatomies. They are quick to anger, unpredictable, and dangerous"
Elswyre is a chaotic place. The north is in a perpetual power struggle over watering holes, trade routes, ariable land, ect. while the South doesnt have a solid group of leaders other than the Mane. Add into the equation a well entrenched mafia-esque drug lord(s) and peace and quiet isn't likely any time soon.
And in consideration for their pride, are they just trying to sort of lord over Tamriel in their own sneaky Khajiit way?
"Until relatively recently, the nearly constant insurrection and tribal warfare among the catmen rarely troubled the stage of history"
The Khajiit arent concerned for Tamriel, it seems. They just want their sugar and to be left alone.
Moon Sugar does not have an intoxicating effect on Khajiit. They eat it every day to no ill effect. Only processed Skooma makes them high, and most of them don't use it. This is from the new novel.
"This sugar has a variety of uses; it is alternately a seasoning and a magical ingredient, a source of communion with the holy moons and a dangerous and addictive drug"
"Humans, it seems, are even more susceptible to the effects of the moon-sugar than the catmen themselves"
"By partaking of the sugar, the khajiit believes they are consuming small portions of their gods' eternal souls. This drives them into fits of ecstasy and abandon, and the streets of Elsweyr's major cities are full of catmen shivering in the grip of sugar fits"
They use it daily and therefore have a high tolerance but it does get them high apparently