Marching with the Gods

Question: Did the Dodekatheon and the Nemetondevos physically fight it out during Ceasar's campaigns, or are the Nemetondevos just upset that the Romans were better at this whole war thing? (Similar question with Shen and Amatsukami and WW2?)

I see you, there, sneaking in two questions for the price of one. You're not fooling anybody.

The Nemetondevos and the Dodekatheon probably did not directly fight much over the wars in continental Europe; just as in the Trojan war, it's more likely that they wielded their mortal champions and seldom interfered personally unless things were looking dire. Fatebonds were still a problem then as they are now, after all, and killing other gods is usually bad for business anyway, considering the Avatar rampages that follow (especially killing them in the World!).

That doesn't mean they were entirely absent, however; in particular, one Roman account of a decisive Gaulish victory claims that they lost because a couple of the Gaulish gods came down to personally kick their asses (so it's not our fault, guys, honest), and of course the Romans in turn claimed that the gods were behind their own unassailable victory over the Gaulish territories later. Just because they weren't there in person doesn't mean they weren't involved; dudes like Julius Caesar and Vercingetorix are prime examples of Scions or chosen mortals that the gods might choose to work through.

Considering the Nemetondevos' habit of being very close to their people, their furious upset with the Dodekatheon probably has more to do with Gaul being subjugated, Romanized and taken away from them than it has to do with any personally-traded slaps.

The Shen and the Amatsukami, on the other hand, are a different situation; I would not assume that any of them were directly involved in the Sino-Japanese wars or World War II, and in fact they seem to have been conspicuously absent (for example, the kamikaze pilots referred to themselves that way in reference to Hachiman having prevented previous fleets from invading Japan, but in a tragic irony this time he did not do so). Considering that it's very close to the modern era and certainly well past the point that the gods entered into their hands-off policy on the World, I would imagine that they did not get their hands dirty (though you'd better believe that the Amatsukami probably screamed bloody blue murder to anyone who would listen once the atomic bomb hit and severely damaged half their ancient territory).

In general, if there's a myth of a god being directly involved, then huzzah! He or she totally was! But if there isn't, it's usually better to assume that they weren't, or were working from afar or through intermediaries (unless, of course, you have a totally rad story plot that involves them being involved, in which case fire away).