Well, unless something extraordinary happens or some unexpected factor/player comes into play, Nilfgaard is far more likely to win than the northern kingdoms
Stuff favoring Nilfgaard:
- Larger army than all of the north combined and they can most likely take a few setbacks, because they have reserves, while the north has to put all cards on the table from the get go.
- Beforementioned army is also superbly equipped and trained, as in professional soldiery, and this standing army has the logistics necessary to actually get into the fight without losing 2/3rds of it´s soldiers to starvation
- The north is not united, the only really strong alliance is either, depending on your choices in TW2: Redania and Temeria if Temeria gets under Radovids thump. Or Redania and Keadwen if Henselt wins Upper Aedirn and forges an alliance with Radovid. These alliances can only muster about a third of the military strength of Nilfgaard and this only if they go "all in", all cards on the table, Win or Die... And then there is the issue of northern monarchs just surrendering without a fight, like all the now client kingdoms in the south.
- Nilfgaard is a relatively stable society with a centralized government and, except the many client kingdoms, the entire army belongs to Emhyr, unlike in the north where half the army is loyal to the various lords and dukes. The only exception is Redania, because Radovid has curbed the "rights" of the nobility and centralized the Kingdom, which is why Redania will be leading the fight against Nilfgaard.
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Things potentially favoring the Northern Kingdoms:
- They are the defenders and are fighting on home ground, thus they will have an easier time supplying their armies.
- The Lodge of Sorceresses if Radovid doesen´t butcher them. Magic is one of the "unexpected factors" mentioned before...
- Disease might affect the Nilfgaardians, throwing the entire operation in jeopardy.
- The Wild Hunt could be a factor in the upcoming war...
- Lastly, Geralt is not a fan of Nilfgaard, but then again, he also recognises that stability is preferable to chaos, even if it comes at a price.
Sorry for my over extensive strategic military historical analysis, it´s kind of my thing