When is a red line not a red line?

The trouble with issuing a threat that if a red line is crossed action will result, is that you have to keep your word or lose credibility.  Pres. Obama has stated several times, most recently during his trip to Israel, that if the Syrian regime uses chemical weapons in Syria, then he will consider that a red line and a game changer.  Earlier this week the Head of IDF intelligence Gen Shlomo Bron stated publicly that a chemical weapon, namely Sarin gas, had been used by the Syrian regime against civilians.   At first Secty. of State Kerry was sceptical about Bron's claim, then Secty of Defense Chuck Hagel visiting the region also expressed doubts. 

But, a day later in light of video from Syria showing civilians clearly suffering from a gas attack, he changed his mind and said that US intelligence agreed that the regime had indeed used chemical weapons. There was also a report some time ago that British intelligence had detected Sarin gas in soil samples taken from Syria.  Also journalists in Syria have reported that they have seen and interviewed patients suffering from chemical gas and also have spoken to doctors who treated them and physiological samples have been tested positively for Sarin gas by US intelligence.  In light of the evidence it is fairly clear that the regime in Syria is using Sarin gas against Syrian civilians.  So what is Pres. Obama going to do?

As of now Pres. Obama has issued a statement saying that the US must receive definitive evidence before any action can be taken.  To many this seems like prevarication and worst it sends the Iranians a bad message, that red lines are not really red lines if the President doesn't want to take action.  Such a show of weakness puts the threats against the Iranian nuclear weapons program in doubt.  It may be that US public opinion is not in favor of US action in Syria.  Although the allies, Britain and France, are supporting the US in regard to action in Syria, PM Cameron said that action could be diplomatic rather than military, but that is really evading the issue. 

Republican Sen. John McCain is calling for a safe zone to be established inside Syria for the Free Syrian Army and Syrian civilians and the protection of that area by declaring it a "fly free" zone, in other words protecting it from Syrian air force attacks.  The White House is using caveats such as only a small amount of Sarin gas was used or they need to assess the overall situation, andfthe Russians are warning against any western intervention in Syria, but these seem like lame excuses considering the preponderance of the evidence.  So when is a red line not a red line?