Take this for example: Two nations are fighting a war on the most ridiculous thing ever. One says that 2+2 is equal to 4 while the other nation says that it is equal to 3. Who is right? That's a dumb question to ask, we KNOW who is right: the country that says that 2+2=4. Are they completely right? Of course they are! Does something like this ever happen? Probably not. But it proves that someone can be completely right.
Now, with regards to time. Things are right no matter what, no matter what passing of time, no matter what we say differently. If something changed from being right to wrong or vice versa from one day to the next then we can say that it was never right in the first place. Take 2+2 as the example again. Did that change from one day to the next? Did it change from one generation or century to the next? It didnt and it cant because if it did then it wouldnt make sense: it wouldnt be right.
Another example? The fact that the earth orbits the sun. We were wrong hundreds of years ago when we said that the sun orbits the earth. Did that change the truth? Did that change the fact that the earth revolves around the sun? Absolutely not. The truth, things that are correct, does not change over time. If it did then it wouldnt be the truth. Whether we realize what the truth is, what right is, what is correct does not change the fact that it is true, right and correct.
How do we figure what is right or wrong? We must obviously have a moral compass otherwise we wouldnt be asking the question in the first place. If we didnt care about what was right or wrong then we wouldnt worry about figuring at the difference between the two. The the thing is, we do think about it which means that a difference between one and the other must exist. Is murder right? Is it?! I would think not! Was it right 2000 years ago? I dont think so. How about 500 years ago? I dont believe so either. Can you kill someone in some cases? Self-defense? Yes, that would make SENSE. Is war then a huge no-no? Depends on what youre fighting for. Sometimes countries are both right, both wrong, one right and the other wrong; we may or may not know. If killing is wrong then that means that war is not permissible but thats what strengthens the argument: mistakes. We made a mistake going to war but a mistake can only be made if there was something RIGHT that could have been done. You cant commit a MISTAKE if there is no right or wrong because that would mean that a mistake is arbitrary and senseless, but it's not, meaning that there IS a difference between right and wrong.
If it is a trivial fact like, "Do trees exist?" then yes, one can be right. But with other things, it really depends. There are different reasons for different things. One side might have a really good reason but have bad reasons with it. Like I said, it really just depends.
When we look at topics like the death penalty, some would say that killing a murderer is justified where others may say it is just as wrong as the murder itself. This would be an example of a grey area. On the other hand, if a person walks up to a child, takes out a weapon and kills them for no good reason, it is clear to all that this is wrong.
Sometimes, war is no different. Look at the U.S.'s involvement in WWII. At the time, the Axis powers goal was world domination, this included the U.S. They also attacked the U.S. and/or declared war on us. This meant that we where clearly in the right to go to war. Had we not, odds are that Germany would probably have succeeded at concurring Europe and Japan would have probably landed on the west coast of the U.S. Eventually, we would probably face a war on both our coasts as well as our north and south borders leading to our eventual destruction. Granted, the use of Nuclear bombs by the U.S. was a bit of a grey area. One thing that does justify it was the fact that Japan refused to accept the defeat of the axis powers and in order to end the war they either had to deploy the bombs or attack them on their land causing even more casualties on both sides including far more civilian casualties.
This is the wonderful world of subjectivism where everyone thinks they are right and are completely justified in thinking this as they have reasons to back it up. However, there are certain things that are right as they can be validated scientifically and be shown to be true over and over again. Just think of gravity, if you start a war as you think gravity is false, then you are demonstrably wrong.
All throughout history our moralities have changed and the reasons we fight have changed. Its safe to say because of this and the controversy that most people at least dont know right from wrong. And the chances of someone being completely righteous are slim to none as we make so many decisions on right and wrong everyday and since there is so much controversy on moral matters. If we knew right from wrong we would be able to explain how something is right or wrong. Thats another fact that tells me we dont know right from wrong. Everything we call wrong today was once called righteousness and everything we call righteousness was one time considered to be wrong. So why fight wars for whats right when we can look back and see everyone fighting because they think they are in the right when we today know that they were wrong. Do you not think future generations will look back and think the same things about us? America isnt perfect... So whats the point of fighting wars for whats right when none of us are completely in the right. Havent heard anyone say how wrong you can be before your a bad guy and any rational person can see that we are all bad guys to an extent so why punish someone for being wrong when we are wrong too.
The boundary between "good" and "evil" is blurred. Someone aligned with evil can be doing something that in their perspective, is good. It all depends on your point of view and side on the fight. To most Nazis, Hitler was correct and reasonable, but at the perspective of an American citizen, he is terrible and not to be worshipped. Good and evil is something created in the minds of humans and it does not truly exist.
There is an ancient proverb that goes something like this: Even if you cut away at a coin a million times, there will always be two sides(at least).
This means that there are good and bad aspects of everything. We also have to take into factor that "History is written by the victors".
For example, look at WW2. We always whine about how the Holocaust was horrible and the evilness of Hitler. And it was. But does that make the Allies "right"? When we talk about how Hitler was racist, we don't think about how the US discriminated African Americans in that same time period.
My point is, yes, there are wrong sides in war, but that does not mean that there is a side that is completely in the "right".