The tenth advantage of democracy: peace. Democratic countries do not fight wars against each other. This is true today, was true in all of the twentieth century, and was true in the nineteenth century in that countries with then democracy-like institutions did not fight each other. A more democratic world would promise to be also a more peaceful world.
The observation that countries in which governments are under some form of popular check are less likely to be warring, was first made by the German philosopher Immanuel Kant in a publication of 1795 entitled Zum ewigen Frieden (Perpetual Peace). Here he not only proposed the equivalent of a UN Charter in which countries commit themselves to peaceful coexistence. He also recommended that countries should adopt republican constitutions since that would make them less prone to war.
The peaceful inclination in democratic governments is due partly to the distribution of power in the population. Since the glories of war accrue mainly to élites and the costs of war fall disproportionately on the populace, élites may incline more to war where they are not answerable to the populace and be more restrained from war where they are under popular control. Other reasons may be that democratic leaders and citizens learn the art of compromise, that they see people in other democratic countries as similar to themselves, and that their communality encourages a habit of peaceful negotiations and treaties.
The danger of war under non-democratic government is currently in evidence in Russia and China. Once Putin had dictatorial control at home, he felt able to go to war against Ukraine. Xi Jinping has ratcheted up war rhetoric against Taiwan (and annexed territory in the South China Sea) as as he has tightened his dictatorial grip in the mainland and Hong Kong.
A qualification: Democratic countries have not in the same way been able or willing to avoid war with non-democratic countries. They have fought wars of more or less defence against non-democratic aggressors, as in the Second World War. But they have also fought wars of aggression in self-interest, as for example the many and violent colonial wars that for example Britain and France engaged in during the nineteenth and twentieth centuries, including Britain’s atrocious Opium Wars of state sponsored drug running against China.
For more detailed analysis, see How Democracies Live.