All of the "red line" debate seems to assume Assad has been proven to have used poison gas on civilians.
Where is the proof?
There's some speculation Saudi Arabia supplied the gas used in the most recent crime:
"As the U.S. considers a response to what it calls a chemical weapon attack by Syrias Bashar al-Assad regime that killed hundreds of civilians, reliable Middle Eastern sources say they have evidence the culprits actually were the rebel forces trying to take over the government.."
"On Aug. 23, LiveLeak.com hosted an audio recording of a phone call broadcast on Syrian TV between a terrorist affiliated with the rebel civilian militia Shuhada al-Bayada Battalion in Homs, Syria, and his Saudi Arabian boss, identified as Abulbasit. The phone call indicates rebel-affiliated terrorists in Syria, not the Assad government, launched the chemical weapons attack in Deir Ballba in the Homs, Syria, countryside.
"The terrorist said his group, which comprises 200 terrorists escaped from al-Bayadah to al-Daar al-Kabera through a tunnel, needed to buy weapons to attack Homs.
"The Saudi financier, who was in Cairo, asked the Syrian terrorists to give details about his group and how it will receive the money. The Saudi admitted his support to terrorists in Daraa and the Damascus countryside. The Syrian terrorist told him that one of the achievements of his 'battalion' was the use of chemical weapons in Deir Ballba.
"The recorded phone call disclosed the cooperation between two terrorist groups in Syria to bring two bottles of Sarin Gas from the Barzeh neighborhood in Damascus.
Evidence: Syria gas attack work of U.S. allies