It is a common trope, yes. It is also an idiotic one and most of the times a weakpoint in the plot.
Only if it isnt handled right, just like any other plot point. The idea that there are no good plots about a weaker force overcoming a stronger one is rediculous. Almost no fiction story has the good guys be stronger than the bad and then win. And there are a lot of good fiction stories out there.
You know what is able to defeat a strong, opressiv force? A stronger force.
Your right. I forgot that tactics play absolutely no role in a fight at all.
How do you want to force the Reds to accept anything? To defeat a nation you need to destroy their armies and/or cripple their economy (which basicly ends up being the same tbh). This is well beyond the scope of Steele.
There are many ways. On way would be to make it too painful for the reds to keep fighting. Both sides are exhausted at this point. Interup supply lines, invoke riots, and like I said before, attack the trench wife situation. Thats a huge potential weakness if exploided properly. Again, you don't have to completely destroy them. You just need to make it so that continuing the war as it is is not worth the effort. THAT does not require a superior military force.
Or, if your Steele is more evil they could develop a way around the reds defense against the golds chemical weapons.
Or, if you are more subtle you could manipulate the reds into pissing off the UGC such that they support the golds. Either through tricking them into doing something or fabricating evidence. Or if the UGC doesn't want to get involved you could get them to just leave like they do if you nuke the base, then broker a deal with mercenaries or some big company that says the golds will give them exclusive rights to something they make, like tit honey or chemical weapons or what have you so that they supply the golds with some men or equipment. The people on myrellion are not technologically advanced, so it wouldn't take much from far more advanced people to put the hurt on them. Again, they don't need to be completrly destroyed, just pressured into giving up on their need for fighting the war. Which probably won't be THAT hard to do considering its been going on for 7 years.
Maybe that scientist who is working on making mods out of red DNA finds a way to neutrilize the effects of red venom and can be convinced to share that with Steele.
All of these are, again, off the top of my head. Imagine what would be possible it given actual thought. There are no limits to what can happen in a fictional setting. And it can certainly be believable if handled well. Like Nonesuch said, it doesn't have to be the golds completely destroying the reds. They just have to be pressured into bringing an end to the war and accepting the golds demands.