What's a Good Way to Introduce Mass Combat?

[VonKatzen]

Of all the campaigns I have pitched so far my Thennla game seems to be getting the most interest on Roll20. Unfortunately I do not have the Taskan militia campaign arc from Age of Treason, as that could be a perfect excuse, but in the current game I intend to tie in to certain military operations taking place in the north of the Taskan Empire.

Assuming the players are the 'adventurer' type, what is a good way to get them at least marginally involved in a battle? Being 'adventurers', though essentially small-time mercenaries, means they're probably not interested in spending the next six months in Kitan eating dried mutton and collecting a feeble stipend. What's true of merc life is doubly true of military enlistment!

Related to how to bring it into the campaign, what is a good way to set up battles so that players are invested in (rather than mere stuck in the middle of) its outcome? Something that comes to mind would be drawing on their Passions, with some object or objective related to a Passion being threatened either by the opposing force or perhaps being held hostage in order to secure cooperation!

I do not intend to run a mercenary or military campaign by any means (that's for my Burning Wheel game set in medieval France) but I do like throwing in elements of martial panoply and formation that have no real role in the small-unit tactics of most adventuring bands.