Down the millennia Buddha, Christ, Muhammad et al couldn't establish lasting peace in the world. Ashoka's and other examples of sporadic peace are nothing but blips on the sand of time. Everything is governed by the absolute law of impermanence. So wishing for permanent universal peace will, as always, remain a 'wish' -- ever beckoning to be fulfilled in some uncertain future, which will never be attained. Don't think it verges on pessimism; rather it's a sincere admission of stark realism.
So to establish peace we have to constantly wage war. That may sound paradoxical but we can't escape from doing so as it is being done in Syria, Iraq, and Palestine currently. Accept that it'll always remain inevitable.
Whatever is happening in the world today bears living testimony to the grim fact.