Yuezhi Invasion

Does anyone know what the triggers are for the Yuezhi invasion. Every time I play as an Eastern faction I live in fear that I will inadvertently trigger the "largest army of horse archers ever seen". Maybe its more realistic not to know, but I'd still appreciate it if anyone would tell me. Thanks.

Uh, another thread asking for triggers. And I expected screenshots and experiences with it.
And sorry, I don't know, but someone will answer soon. (And when you experience it, please, post some screenshots)

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This...
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I don't think it was so bad fighting them. For some reason the four stacks don't reinforce each other, so take them out one at a time.

182bc in my Pahlava campaign... I hold Chighu and would like to finish the Saka before They come. I should learn to use screenshots.

Uh-oh. Thanks everybody for the responses, but it looks like I'm in trouble. I'm in 196 BC, playing as the Pahlava. I owned Chighu about twenty years ago, but it's Eleutheroi now. Still, my problem is that I'm all tied up fighting the Ptolemies and what's left of the Seleukids in the west. Guess I'll have to free up some stacks and sent them east for the emergency. Keeping fingers crossed.