Multiple clan battle question

Quick question about how S2TW handles multiple clan battle setups in the campaign (SP or MP head-to-head).

Let's say that Shimazu and Uesugi are at war.
Uesugi is also at war with the Ashikaga, but Shimazu is not (though neither are they allied with the Ashikaga).

Uesugi attacks Kyoto, but within 'reinforcement' range is a Shimazu army.

Does the Shimazu army get included in the battle? If so, is it in support of the Ashikaga (despite having no alliance), or independent of either other faction? I vaguely recall that MTW or M2TW had some feature where you could actually declare war on another faction right in the battle (and even betray your ally with force-attack) but I'm curious as to whether or not this made it into S2.