900s - 1100s, possibly even as early as the 800s.
http://hbdchick.wordpress.com/2013/11/25/kinship-the-state-and-violence/
If there's any large organization you can credit for clannism (and hence violence) reduction, it might actually be the Catholic Church, because they were the ones with the biggest and earliest hardon for breaking up clans. Then again, nobody yet knows why it was the Dutch and the Anglo-Saxons that followed the Church's ideas so faithfully ...
A pair of links I found recently (via Marginal Revolution) and haven't found on LW:
http://www.cato-unbound.org/2014/03/10/mark-s-weiner/paradox-modern-individualism
https://bitcointalk.org/index.php?topic=475054.0;all
The former discusses liberty in the context of clannish behaviour, arguing that it is the existence of the institutions of modern democracies that allows people individual liberty, as it precludes the need for clan structures (extended family groups, crime syndicates, patronage networks and such).
The latter is a author's summary of a white paper on the subject of decentralised Bitcoin prediction markets with a link to the paper.