Your question is loaded with too many assumptions. You assume international laws as they stand are good as they stand, you assume torture works to obtain information, but more than that you assume these forces exist in a vacuum. By your logic and with your assumptions obviously evil is more powerful but you've papered over so much that the answer to your question is meaningless.

There is also the materialist part of your problem which is it assumes both sides have access to the same resources (and that they work the way you think they do). How many more Good people are there than Evil? Do the Evil people have the knowledge and skills to produce the weapons to facilitate the tactics you lay out or would they rely on Good people to produce them? If they do then what happens if the Good people object to making the weapons that facilitate their winning tactics? How can the Evil people conscript the Good people into performing their duty without those weapons?

Finally it assumes states act in unified ways under central control, and that everyone in those states are state actors and act under command of the state. There is no country in the history of the world where that has ever been true, not only does it go against the concept of free will (which your name suggests you don't believe in) it also assumes that either states are conscious entities of themselves or there are conscious people controlling these states and every single person in them or acting on their behalf.

'Is the Dark Side stronger?' 'No, no, no. Quicker, easier, more seductive.'

Lets say there are 2 factions at war. One Evil and One Good.

That's a ludicrously strong assumption. Even the weaker assumption that good and evil are even well defined is still too strong for practical purposes.

There is simply no such thing as "good" or "evil" which can be canonically defined without reference to some expected value assigned to the human experience at a bare minimum. Any reasonable definitions of "good" and "evil" require at least one nontrivial, nonphysical, subjective value judgement to be fixed as part of the axioms of the ethics that define them.

Evil people can just ignore international laws and commit war crimes, Good people will have to abide by laws.

In my view, good people ignore international laws all the time, such as migrants crossing into countries illegally. Evil people often enforce international law...such as those who enforce laws banning migrants from crossing international borders. Notice that I've made the value judgement that borders are stupid [1]; there's no physical reason to indicate that migrants crossing borders have any moral relevance.

Country B: Half Good Half Evil

This requires the even stronger assumption that good and evil act like numbers, e.g. things you can add and get a predictable result, or take half of. Now I'm an engineer and math... enjoyer... so I get how convenient it would be to work with numbers. However, good and evil cannot be measured, and we have not found a mathematical model that adequately or unambiguously models the goodness or evilness of an act or person as a number or more generally a field. The law of unintended consequences suggests that approximating ethical decisions as operating on a number or number-like object doesn't yield reliable results in general.

The rest of your arguments rely on further stronger assumptions about how people "would" act. People are complicated, unpredictable, time-varying, and inconsistent. It is difficult to predict what people will do, and for this reason you need to supply additional facts to establish a cause for why people will follow your assumption. For your points to be valid, there needs to be either some historical evidence or at least a heuristic justification under much weaker assumptions when evidence is unavailable.

Said differently: your argument is fine (I think), but your premises aren't true in general, so your conclusions based on them cannot be true in general.

[1] Actually, "borders are stupid" is an unnecessarily strong assumption, because I could start with much weaker assumptions, such as "human beings are more important than the laws that govern them" and then deduce that borders are stupid from history and a minimal set of axioms. But this comment is getting long enough, and I honestly find ethics to be kinda dry.

The point of evil is it defeats itself. Suppose you're evil and you want to scare your enemies into not killing your messengers you send. You might spread around the idea that "the gods punish those who kill the messengers". Then suppose, again, you're evil, but this time you want to express that by being brutal to your enemies. When they reach out to you, you might then kick the messengers down a well to show your ego. Congratulations, you now have scared your citizens into thinking you're all cursed.

In other words, evil is the name for something that runs on double standards. And yes, my example is inspired by Sparta. Would you like to know what happened next? Sparta became so afraid of the curse of the gods that they sent messengers to the Persians and asked the messengers to beg to be killed for retribution... only for the Persians to be better than their enemies and treat the messengers well. People forget Spartans and ancient Greeks in general were hypocrites.

If you are too attached to abstract ideas of "Good," to the point of standing on principle even if it means defeat, then you may get defeated and replaced by someone worse, and things may be much worse for everyone if you did whatever was necessary to win.

However, if you are too attached to the evil approach, then you may become too caught up in self-indulgent antihero fantasies and lose sight of what actually works. For example, no actionable information was acquired through the use of torture during the War on Terror. Movies and TV shows almost always depict it working because that self-indulgent antihero fantasy sells, but that doesn't mean it's true. Violating international law could also (depending on the rest of the factions in this hypothetical) cause diplomatic repercussions such as sanctions or even another faction joining the war on the other side. Evil doesn't always get away unpunished.

Every faction, (as well as every person), has a variety of different approaches that they could apply, and what determines which of these approaches work and don't work is the system in which they exist and their position in it. "It is not the strongest of the species that survive, nor the most intelligent, but the ones most adaptable to change."

Not more powerful, just more willing to do whatever to get what they want.

If good people always lose then why do we still have good people? Why do good acts still occur?

Counterpoint: Goku always wins and he’s the good guy

Wars tend to involve civilians getting hurt, because yeah, it’s cheaper and easier to disregard international law.

I wouldn’t generalize that to evil always winning vs good, though. Human life is complicated, and mean, but progress gets made anyway. There’s a reason most people dislike war.

Good triumphs not due to actions but because we want it to.