After the 20th century's list of events of mass murder — from the Ukraine famine of the early 1930s and the Holocaust in the 1940s, to the Balkans wars and the Rwanda genocide of the 1990s — the cries of "never again" and the assertion of a "responsibility to protect" gave some hope that mass killing would not recur in the 21st century. Then came Darfur in the new century's first decade, and now Syria in the second. Mass killing has very clearly not been eliminated, nor has the "international community" developed a response that will avert it or bring it to a quick end.
Simultaneously, Syria has become the test of another international agreement: to outlaw the use of chemical weapons. It is clear that the Assad regime used such a weapon — sarin gas — time after time, and on that basis the Obama administration prepared a military attack to punish the regime last summer to deter and prevent further use. At the last minute it turned from that attack to a deal, negotiated largely via Russia, under which the regime promised to eliminate its chemical weapons arsenal and never again to use chemical weapons. What portion of its chemical weapons it has in fact shipped out is open to debate, but the regime has been widely reported to have used a different form of chemical weapons repeatedly in 2014: this time, bombs laced with chlorine. So like mass killing, use of poison gas turns out to be a feature of this new century as well. If we face a new century in which the use of poison gas and mass killing recur regularly, Syria will be the place where they reappeared — and the policies of Barack Obama will be a central reason why they have returned.