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Being nobody is sure that the Syrian army was responsible for the Ghouta sarin gas attack in August 2013, how can we be so sure that this same army was responsible for the recent gas attack? It could have been Assad's army or the rebels, the more important question is, who supplies these people with these chemical weapons? As far as I know Syria can't produce them, so someone is supplying either Assad or the rebels, shouldn't that someone be held at least in part responsible for these tragedies?

Syria has been producing chemical weapons since the 80's. Look it up. They area a rich nation, with absolutely the capabilities to maintain a stockpile of chem weapons. These are facts.

I did look it up, all it says is "it is believed", another one says that Syria is were Saddam hid the WMD, George Bush didn't find in Iraq. The only one I could find that was sourced was this:
The US stated in 2002 that Syria had a "long-standing chemical warfare programme", which was first developed in the 1970s. A recent report from the US Congressional Research Service said Syria probably began stockpiling chemical weapons in 1972 or 1973, when it was given a small number of chemicals and delivery systems by Egypt before the 1973 Arab-Israeli war.
And frankly if it is the same source that put the weapons of mass destruction in Iraq, somehow I am not convinced.
Anyway, I'll ask you something, no matter if it is true that Syria produced chemical weapons or not, how can the US be so sure that Assad's people were behind this attack? The rebels are supposed to have them also, and they are the ones who gain the most if the US attacks Assad.
Look this is certainly a no win situation for everybody, on one side you have a totalitarian despot, on the other you have very militant and frankly crazy rebels, then you have the US getting involved with Russia there, and the scenario doesn't look good.

"In September 2013, Syria provided information about its stockpile to the OPCW as part of its disarmament obligations. However, the exact composition of its declared chemical arsenal will not be disclosed to the public, due to OPCW rules.[16]" -Wikipedia (number for source)

Syria has publicly admitted to having chemical weapons, but had said they had destroyed them. The OPCW is the Organization for the Prohibition of Chemical Weapons, an international organization.

As to the US's response, I am in complete agreement with you. I don't really believe that the US can impact anything positively in the ME. Nobody wants us there, not our "allies" or the people we are claiming to help. It's a lose-lose scenario.

Attacking Assad will only prolong the war and lead to more death.

They could also have bought them, there are enough money hungry companies and people in the world willing to sell these weapons, I believe there is a black market for them so anything is possible, but it is also possible that Syria was producing them as you say so let's leave it at that.