I'm a peaceful activist who believes we must significantly disable Syria's military. Poison gassing is an atrocity, on top of 100,000 horrible atrocities, that must receive severe consequences. Without consequences, no standards against abhorrent behavior can be upheld.
Objectors say, "why should we be policeman to the world?" The answer is we are the most powerful country that also stands for principles of human rights and justice. True, we sometimes give only lip-service to these ideals while pursuing selfish national or corporate interests, but, the other powerful countries don't even pretend to uphold these ideals. They'd let people kill each other, so long as they can buy resources and sell their products. If we don't act, who will?
Objectors say, "why go to war now when we did nothing while 100,000 civilians have been killed already?" The answer is that we've no clear threshold beyond which condemnation and reaction are called for. Should the world have reacted after 100,000 murders? or 50,000? or 1,000? or 50? How many people must die to call this an atrocity? We don't know. But we do know that use of poison gas, biological or nuclear weapons IS an atrocity, and now, that line has been crossed.
How can peace-loving people condone war? When negotiation and diplomacy fail, then one must push back when confronted by a bully. Diplomacy only works when all parties actually want to find a resolution. In the case of Syria, the tyrant Assad wants only to eliminate his opposition.