I am aware that one should never underestimate the power of human stupidity, but I find the theory that the Syrian war was initiated over planned gas pipelines to be somewhat implausible. At the very least, if this was the reason for America's involvement in Syria, it was an exceptionally stupid one.
First of all, Syria is absolutely not needed to get gas from Qatar to the European Union. Instead of pumping gas from Qatar to Saudi Arabia, from Saudi Arabia to partially unstable Iraq, from Iraq to Syria, from Syria to Turkey and then from Turkey to the EU, you could build a gas pipeline from Qatar through Saudi Arabia and Egypt directly to mainland Greece. (Like Qatar and Saudi Arabia, Egypt too has a population that predominantly adhers to Sunni Islam. So, even the religious compatibility angle, as referenced by Robert F. Kennedy Jr, appears to fit in this scenario.)
While this would involve building an off-shore pipeline through the Mediterranean Sea, this is hardly unprecedented. Russian Gazprom built the 750 miles long Nord Stream pipeline under the Baltic Sea. The off-shore part cost roughly 9 billion EUR. The project was so successful, they are actively planning to build a second pipeline right next to the current one. Gazprom also build the Blue Stream pipeline under the Black Sea. It transports natural gas from Russia to Turkey and is about as long as Nord Stream (750 miles).
For comparison, building an off-shore pipeline that connects Eygpt to either Greece (or Turkey, if preferred) would have to be a mere 350 miles long. If we use the Nord Stream costs as a guideline, the off-shore part would likely cost around 4 - 4.5 billion EUR. That is not an excessively high price tag, especially if it allows you to reduce the number of involved countries, which generally lowers potential investment risks in a region that is not known for political stability.
Secondly, if one were to conclude that it would be against US interests if there was a pipeline that connects Iran to the EU, it should have been relatively easy to persuade Turkey from blocking the project. At the time, the US and Turkey were still extremely close. (The Syrian war started years before the US moved its nuclear missiles to Romania.) Also, the US would have received - direct or at least indirect - support from Russia who would naturally prefer if Gazprom remained the only company with cheap access to a major natural gas pipeline in Turkey, and who has no interest in new competition from an emerging natural gas exporter.