The Peace of Westphalia ended the 30 Years War, a sectarian religious conflict that ravaged Europe in the mid seventeenth century. It established a principle of international law persisting to this day that Mr. Hague would do well to revisit. A sovereign state may not suffer military intervention from another state on account of internal domestic conflicts. It was the lack of such a tempering principle that spread sectarian violence throughout Europe at the time.
So, no matter how horrific the events inside Syria, proxy wars on behalf of combatants always spill over into regional conflicts (don't feed the terrorists) and can even lead to wider international conflicts. Remember that we declared war on Germany only when it invaded Poland. However, I am NOT saying that we shouldn't have done so if Nazism had confined itself to internal German affairs rather than try to build the Third Empire. Systematic mass killing and genocide must always be resisted BUT (and we should take lessons from Iraq and WMD) only with due authority (one of the priniciples of a "just war.")
Once again the US and the UK are beginning to act alone. Short of a UN declaration, the Syrian government is the legitimate authority in Syria. If there is a credible alternative to the present regime, colluding with armed insurrection and risking military hardware falling into the hands of jihadists is not the way to provide it.
Yes, it's horrible to see so much suffering in Syria but the choice is whether we want to see yet more suffering by external as well as internal escalation. The west needs to work with Russia in bringing the combatants to the peace table. If Russia is arming the government rather than the militias, however unpalatable that might be to some, it has international law on its side. This is not about sentiment but rather a hard nosed assessment of what will lead to a cessation of the violence and, thereafter, contribute to the long and hard business of healing wounds and building peace.