In the summer of 1864 as the US General William Tecumseh Sherman surrounded the Confederate city of Atlanta, Georgia, he received a letter from the city's residents pleading for him to spare their homes from his scorched earth campaign through the rebel South.

Sherman wrote back:

You cannot qualify war in harsher terms than I will. War is cruelty, and you cannot refine it.

Only by terrorizing the people of Atlanta he argued, could the end of a war that had by that point taken so many young lives be hastened. Days later Sherman was true to his word. Atlanta was burned to the ground. In the 150 years since Sherman's statement about the harsh realities of armed conflict, war has remained unforgivably cruel. It's also, however, been repeatedly refined.

International treaties and conventions along with an evolving set of global norms have helped to prevent war and limit their deadliness. In that constant process of "refinement" lays the single best justification for President Obama to launch a military strike against Syria for its recent taboo-shattering use of chemical weapons against its own people.

While it won't fully stop the bloodletting in Syria, upholding and sustaining the long-standing global norm against chemical weapons – while sending a message to President Assad as well as future dictators intent on employing such weapons and such tactics – offers a compelling rationale for even a punitive use of force.

Why norm enforcement matters has much to do with one of the most extraordinary and under-appreciated developments in human relations in our lifetimes: the declining propensity for violence and war. While this may seem a counterintuitive statement considering the situation in Syria, war has become a far less dominant feature of the global environment. There were last year a mere six conflicts. Wars between states have basically disappeared. When wars do break out, they tend to kill far fewer civilians. While the reasons for this decline are numerous, the role of international law and global norms has been of critical importance. Quite simply, it's no longer considered acceptable, either by custom or by law, to invade and conquer your neighbors or to indiscriminately harm civilians, and there are multilateral institutions and global legal frameworks to both prevent such atrocities from occurring and also punish the guilty.

However, for such laws and norms to have any validity they occasionally need to be enforced.

For example, when Saddam Hussein invaded Kuwait in 1990, it wasn't the first time that a country had violated international law by attacking its neighbor for reasons other than self-defense. But the US-led war that reversed that invasion not only strengthened the notion that such conquests were forbidden by international law, it created a new recognizable norm in global affairs – namely that such cross-border attacks could no longer be executed without serious consequences. While the connection is perhaps more correlation than causation, it's a norm that in the 20 years since has largely remained sacrosanct.

Chemical weapons falls into a somewhat similar category. The taboo against the use of such weapons is one of the longest-standing global norms. It dates back nearly a century to the First World War and the subsequent Geneva Protocol, which sought to ban their use. The reason was simple, chemical weapons cause horrible, mass deaths and are primarily a tool for harming the innocent rather than waging war. Since then, while man's inhumanity to man has been replayed countless times in countless nations and by countless armies, the prohibition against the use of chemical weapons has generally been maintained.

In fact, one of the more successful global disarmament treaties is the Chemical Weapons Convention, which has put muscle on the chemical weapons taboo by destroying stockpiles of weapons and banning their production. To date 189 countries have joined the convention (Syria, unfortunately, is one of the seven countries that hasn't). While there is no enforcement mechanism for the CWC, the use of chemical weapons is illegal and there is widespread international agreement that upholding this norm is a significant global priority. So norm-enforcement as a justification for the use of force rests on fairly solid ground.

Of course, the obvious response to this argument is "why it is ok to kill Syrians with conventional weapons but not chemical weapons"? If the US wasn't prepared to raise a fuss over the slaughter of hundreds of thousands of people in Syria's civil war why start now?

This is a far trickier issue to defend, but one way to think about this is the same way we think about gun control.

As any gun control advocate will tell you, eliminating all gun violence in America is unachievable. There are too many firearms and too strong a cultural attachment to guns. The goal instead is to limit the number of deaths; to keep guns out of the hands of criminals (and potential ones), to make it more difficult to purchase a firearm in the first place and to limit the damage that one can do (i.e. banning assault weapons or large magazine clips).

Global norms on war-fighting are not completely dissimilar. The goal in regulating war is not to wipe it out completely (that likely won't ever happen), but to make it less likely to occur and when it does happen, ensure that it wreaks less havoc.

That's why new norms and legal frameworks have consistently been developed to limit war-fighting capabilities, with a special focus on weapons that target civilians, kill indiscriminately or enable killing on a mass scale.

The international community has largely failed in efforts in Syria, in part, because global norms on protecting civilians from their own governments haven't evolved to the point where intervention is considered acceptable (and likely never will completely). Still the norms that do exist are important and maintaining them is one of the best tools at America's disposal to further limit and contain violence and not just in Syria, but in future conflicts (of course it would also help if the US abided by them as well . . ).

The fact is there are and should be rules to the game, and when a country acts so far beyond the generally accepted guidelines of armed combat, a response is necessary. That's why, for example, ethnic cleansing in the Balkans or more recently boasts about killing thousands in Benghazi led to outside intervention. While neither of these legally qualified as genocide, which under international conventions can merit a military response, they were objectionable enough that it led to a strong international reaction. One could even argue that such responses helped to create new global norms of their own.

This doesn't mean that the answer to norm violations has to always be the lobbing of cruise missiles or that every provocation should or will garner a military reply. Indeed, the strongest argument against the Obama Administration's plans is that no non-kinetic punishments (like a referral to the International Criminal Court) appear to have been considered. Although, since this is not the first alleged incident of chemical weapon use by President Assad's regime, it's hard to argue that an ICC referral would have much of an impact. Then again, there is also no guarantee that a few cruise missiles will deter Assad.

It's these uncertainties and the lack of a clear strategic objective in Syria that makes this situation so difficult.

Do nothing and the US risks allowing the largely, century-long prohibition on the use of chemical weapons to be weakened. Adherence to global norms plays an important role in limiting warfare and this has benefited the United States. Does America really want to be sitting on the sidelines as this process is begun to come undone?

Do something and you risk further loss of life in Syria, an escalation of the conflict, a hardening of the Assad regime's position and unintended consequences that the advocates of the use of force rarely seem to consider. Most ironically, the greatest downside to a US military attack against Syria is that it would almost certainly have to be done without a UN Security Council imprimatur. That means to uphold the norm on chemical weapons it would be necessary to violate international law.

Still if the US response is a limited one (no boots on the ground and no regime change); if it's articulated as a response to the use of chemical weapons; if it's intended to serve as deterrent to both Assad and future tyrants, then there is a legitimate justification for force to be utilized. If President Obama uses the attack as an opportunity to jump-start political negotiations between the Assad regime and the rebels, a punitive attack could potentially play a positive even constructive role in ending the conflict. Yet even if that fails, upholding the global set of norms and rules that have contributed to what is probably the least war-like period in the history of the species is not nothing.