[quote=@Foster]
[quote=@Flamelord]
Yes, but it;s also a major festival. Lots of civilians from around Remnant, people's guard is lowered because they're busy with the festival itself. Think of how much of a propaganda coup it would be to have a major infiltration of the city by Grimm, during what is one of the biggest festivals of the year, in the full view of the world. What does that say about the Vale government and it's ability to defend itself as well as protect it's citizens?

Basically, even if they had been defeated, it would have been a massive blow to the reputation of Beacon (For a real life example, look at the Sochi Olympics and the Volgograd Bombings), and it gives them legitimacy and public awareness. And fear.
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I was thinking more like the Iranian Embassy raid [url=https://www.youtube.com/watch?v=Pjxbtr-VR70]or flight 8969[/url], since the tunnel ended literally at the doorstep of what amounted to whatever training facility the SAS uses, rather than a certain heavy-handed regular army garrison.
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Well, it's not like a bunch of Grimm getting slaughtered is a blow to the cause anyway. It's just a message to Vale that 'hey, we're something you have to worry about'. Besides, add in he White Fang causing havoc, perhaps with those mechs, at the same time, and containing it becomes a lot harder. And if it isn't immediately and efficient handled, even winning can't really be seen as a victory. 

Is it bad that we haven't even seen the actual Vale army at this point? All we've seen thus far are Ironwood's forces, and they're from Atlas, not Vale.