If this is your first time here, I recommend starting from the conclusion page.
The London Review of Books published today a long piece by Seymour Hersh analyzing the intelligence surrounding the August 21st attack. In this post I'll review the main points raised in the article.
Sensors around chemical weapons sites were not triggered prior to the attack
The article doesn't provide any firm evidence that supports this, but it does provide some circumstantial evidence (when they were triggered in December Obama issued a warning, while no such warning was issued in August), and it's reasonable to assume such an early warning system exists.
Since we already know the sarin was probably not from the government's stockpile, and that the rockets were not standard chemical rockets, this new observation makes perfect sense but does not change the picture significantly. In other words, even if this attack was somehow carried out by the regime, it used non-standard weaponry and should not have triggered the alarm.
The US evidence relating to personnel movements and military orders was cherry picked in hind sight.
This again makes perfect sense. There are probably millions of data points collected every day from Syria, which cannot be analyzed in real time. It's safe to assume that following the attack this data was analyzed in retrospect. The fact that despite this effort, the only evidence presented by the US was very weak and circumstantial, is a strong testimony to cherry-picking. Actually, the fact that nothing incriminating wasn't found in all these sources is strong probabilistic evidence that the regime was not involved. This is analyzed in detail here.
The 330 mm rocket (aka UMLACA) is an improvised munition, implying it is not related to the government.
Here Seymour is of course out of touch with the current research at Brown Moses and WhoGhouta, which clearly indicates that the rocket was developed for the Syrian Army, probably to fit its special needs in short-range urban warfare.
However, this in itself is a weak indication of government involvement, since most of the opposition's heavy weaponry has been looted from Army depots.
The rocket's range is less than 2 km, indicating the NY Times report claiming the attack came from an Army base 9 km away is incorrect.
This was already established here three months ago. However, he does say this range estimate was based on "a thorough study", so unless this refers to the WhoGhouta research, it gives yet more credibility to our range estimate.
US Intelligence estimates the opposition has the capability to acquire and use sarin.
This is in line with our analysis, and while no additional hard evidence is provided, his quotes from intelligence sources seem reliable.
Summary: While Hersh does not provide significant new evidence, his quotes from intelligence sources are in line with many of our findings, and his analysis provides much-needed counterweight to the many erroneous reports in the media.