Yeah, lets pick only one war at a time
We are currently at 3 to 4 ongoing simultaneous international conflicts / wars: Red Court, Kakuri/Wicked City, Denarians, Nemesis.
Yes there is.
99% of the setting wont make that threshold. But we're dealing with the 1%, and I suspect that they can make it through the portal if they put their minds to it.
An advanced army has limits when the OPFOR potentially includes entities that can directly strike at your volition.
I dont think they can rampage unchecked through Sanctuary, mind, any more than we could through the Wicked City, but achieving sufficient access to do so enough to strike at Sanctuary's magical topography seems well within their capabilities.
Even if they have to outsource it to like a Walker or an elder Black Court vampire sorcerer with Dominate.
I mean, people forget that, other than Nemesis/He Who Walks Beside, there are currently two Walkers, what this quest would call Great Walkers, physically loose on Earth right now.
==
Making a powerful vampire organically takes time.
Upgrading an existing one with aftermarket support, not so much. Vito Malvora in canon, and Duke Skavis in this quest are two examples of what can be done with a willing vampire and external support.
And the Red Court has an excess of feral Red Court vampires to expend if necessary, and no shortage of new ones.
They are the second fastest breeding species of vampires after the Blacks.
=====
I feel you a) strongly underestimating our forces, and b) strongly overstating the importance of that 1% (actually much less) who might get through.
First, about underestimating:
1) We got into Wicked City via infiltration. This is a prepared hard point. A completely different setup. Very few, if any beings have perfect universal scene long defenses, which is basically what's needed here. Especially after we spend some AP reinforcing the entryway with exalted large scale craft
2) Our home field advantage is immense. In the same way our spirit binders are unfamiliar with earth-side spirit world, so too would the invaders be unfamiliar with the rules of our world. Just as quick examples:
2.1) no NenverNever. It's a material world, but there's no NeverNever access. Hell, potentially no hell access.
2.2) Comlletely alien spirits, spiritual landscape, overwhelming aura of hell, etc.
2.3) Lord of the Land and secondary effects. Wizards often work by calling g upon and manipulating outside forces, like Dresden gathering heat from an area around him. In Sanctuary this is probably going to be an opposed roll against Molly's something, because the energies of the world are essentially her bodily humors
3) Ours is not a mundane army hindered by Masquerade. II's an industrial magitech army the likes of which Earth has never seen, that enjoys intelligence advantage. They know (mostly) what might be coming - we (hopefully) briefed them in detail about threats. The attackers are coming in blind, and will keep coming in blind.
Now, the second part, about overestimating the "1%" significance:
1) Most of those who could plausibly breach and get past the defensive lines, are those tied to a faction. If, for example, Emma-O tries to attack us personally, he risks his rivals attacking him while he's occupied in Sanctuary. Remember - travel time is not instantaneous for most parties in Dresden Files. Travel takes hours, some times days if we go with deep NeverNever.
The same issue applies to Red Court specifically - whilelords of outer night are attacking Sanctuary, they are not protecting their bases against Molly or White Council, and at least Molly has mobility superiority.
2) Denarians might not be allowed to come into our realms. Swords of the Cross generally operate on rules comparable to those that govern Denarians in regards to where they can go and how they can operate. The Swords aren't allowed to enter our realm. I expect coins aren't allowed either, unless we bring them in ourselves. Remember, Sanctuary is, very literally, our soul. Denarians cannot take entirely unwilling hosts, you can put down the coin. Forcefully taking a coin into someone's soul is very likely breaking the rules. Not to mention that, as a celestial exalt, our soul is immune from possession and would likely reject the coin vrom inside it automatically.
3) Nemesis can only operate through hosts. Yes, it can boost them, but that breaks its cover, which is a Los condition for it.