Channeling some heinlein here:
If we were all galapagos iguanas, well, then one of us iguanas will find a way to swim or otherwise get to a larger landmass. Or find a way to live in the ocean.
Then, one of us would find a way to breed faster. After all, there is infinite seaweed and algae available to me.
Eventually then, this adapted iguana will be so widespread, even if it remains docile and somehow does not evolve some territorial behaviour for mating purposes (unlikely), that, docile and all, it eats all the seaweed.
Some seaweed will likely mutate into something that is harder to eat for the iguana, or poisons them and iguanas learn not to eat that (why? Well, there are a ton of iguanas - whatever iguana just mutates their genes into recognizing the poisonous one by taste or smell and won't eat it, will survive where the ones without the mutation will start dying off in droves).
And now we have our war back.
It's just between iguanas and the seaweed, which is now in an arms race: The iguanas adapt to deal with the poison, the seaweed adapts to come up with new poisons or otherwise make themselves harder to eat. The rules have changed: There are so many iguanas, that if you are easy to eat by an iguana, you, as a seaweed, are just not going to survive, because there are so many.
Or, if the seaweed does not manage to be an equal aggressor in this fight, then something else happens: First massive, MASSIVE die-outs of iguanas due to starvation, and then eventually, aggressive behaviour between iguanas: If I manage to find a stretch of beach with a bunch of seaweed, and I just had some kids? Well, I can just stay docile. Or, I adapt to be aggressive, I chase off other (docile) iguanas hungry and on the lookout for seaweed, and thus ensure that my children survive whilst theirs die of starvation.
War is inevitable, unless the system grows into 'acceptance', a self-balanced status quo where opportunities to expand exist but for whatever reason a part of the ecosystem just decides not to do this. For example, what if the iguana's somehow have a break built in and do not adapt. It's that or find a lifeform that requires literally zero resources.
The problem is, given that this is an utopic view where life is great and nothing needs to change and there are no external pressures to adapt, why would you invent anything? Until some external force shows up and changes things, life will continue stable forever, with iguanas doing iguana things and seaweed doing seaweed things.
Heinlein's principle is that this can only hold if you are literally the only life in the entire universe (or all life is like this), because if not, one day one of the ecosystems that decided to 'war' and grow will obliterate you. They don't play by the same rules.