One in which orangutans are humanity's closest relatives, rather than chimpanzees (which are in reality). Homo sapiens did evolve but died out 10,000 years ago. Chimpanzees are one of the most aggressive species of primates, while orangutans are relatively peaceful. This simple fact changes everything.
Though not that physically different, humanity takes a very different course. This hypothetical society advances culturally and technologically far faster than our civilization did in reality. While extremely violent in tribal times due to pressure from the environment, this changes later to a polar opposite extreme. By their 'iron age' period, a concept of the 'Greater Good' (basically various levels of enlightened utilitarianism) has started to take root as a core of 'Human' society. Humans serve their village, city, or nation rather than themselves. Environmental factors render some nations more hostile than others, but things are generally peaceful. These humans would rather starve than hurt others and while they will survive, they put the good of the group ahead of them selves. Again, this dramatically speeds up the 'tech rate' and productivity. Unfortunately, this human race's lack of concept of 'war' ends up fucking them over later. There is no religion, but a sort of 'oneness' ritual and general spirituality akin to Buddhism. Disputes are settled thru convened meetings about what would satisfy the needs of the most people in varying levels of complexity. Long term thinking is a valued attribute. 'Punishment' is carried out generally thru rehabilitation of both the actor and anyone who influenced or put them in the wrong position.
By the technology level of the Roman empire (and there is no collapse, so technological advancement continues to explode) some aggressive nations learn that they can exploit this peaceful if not naive nature. Though their foot soldiers feel disgusted and committing aggressive acts, they are compelled by their leaders anyway. A few small aggressive states end up taking over a quarter of the major 'powers'
'territory' (though both concepts are generally treated as alien). These 'powers' are far more advanced than the barbarians but are helped by a small number of defectors, who convene with the peaceful councils and try to use their expertise to convince the peacefuls to arm themselves. Having both converted to the 'greater good' but realizing the aggression of the 'enemy', they end up seizing some level of power just to simply prevent the main powers from being wiped off the map.
Though the peacefuls try to stop it, the converts launch an 1800s tech artillery bombardment on the 1500s level army of the enemy. It decimates them and sends them running. In an unusual display of aggression, the peacefuls are driven to the point of executing those who ordered the bombardment (which has been treated as the highest order of heinous act) thru painless and humane means. Then, those who executed them are shunned and end up disgraced and in rehabilitation for their crimes. Six months. Rehabilitation is comfortable and primarily centered on persuasion and 'opening the eyes'. However, some converts remain and the peacefuls do learn that they may have been too naive.
A compromise is reached. The peacefuls and the converts develop weaponry that is geared almost entirely towards capturing. Some examples include sticky net launchers and long range 'rifles' that fire an electrified dart. Future invasions result in peaceful soldiers sacrificing themselves to ensure that invaders are captured and not hurt, and those that end up killing an invader (especially when they could have done otherwise) are utterly shamed. Trying to inflict the minimum damage possible is the highest priority. Even hitting someone in the knee instead of the thigh (because it could cripple the invader) is considered an offense to some degree.
well, is it?