I recommend you read the full article; lot’s of information in there. Hersh’s main point is that there are sensor that would have alerted Washington if Syria was preparing for a chemical weapons strike:
The sensors are monitored by the National Reconnaissance Office, the agency that controls all US intelligence satellites in orbit. According to thePostsummary, the NRO is also assigned ‘to extract data from sensors placed on the ground’ inside Syria. The former senior intelligence official, who had direct knowledge of the programme, told me that NRO sensors have been implanted near all known chemical warfare sites in Syria. They are designed to provide constant monitoring of the movement of chemical warheads stored by the military. But far more important, in terms of early warning, is the sensors’ ability to alert US and Israeli intelligence when warheads are being loaded with sarin. (As a neighbouring country, Israel has always been on the alert for changes in the Syrian chemical arsenal, and works closely with American intelligence on early warnings.) A chemical warhead, once loaded with sarin, has a shelf life of a few days or less – the nerve agent begins eroding the rocket almost immediately: it’s a use-it-or-lose-it mass killer. ‘The Syrian army doesn’t have three days to prepare for a chemical attack,’ the former senior intelligence official told me. ‘We created the sensor system for immediate reaction, like an air raid warning or a fire alarm. You can’t have a warning over three days because everyone involved would be dead. It is either right now or you’re history. You do not spend three days getting ready to fire nerve gas.’ The sensors detected no movement in the months and days before 21 August, the former official said. It is of course possible that sarin had been supplied to the Syrian army by other means, but the lack of warning meant that Washington was unable to monitor the events in Eastern Ghouta as they unfolded.
The sensors had worked in the past, as the Syrian leadership knew all too well. Last December the sensor system picked up signs of what seemed to be sarin production at a chemical weapons depot. It was not immediately clear whether the Syrian army was simulating sarin production as part of an exercise (all militaries constantly carry out such exercises) or actually preparing an attack. At the time, Obama publicly warned Syria that using sarin was ‘totally unacceptable’; a similar message was also passed by diplomatic means. The event was later determined to be part of a series of exercises, according to the former senior intelligence official: ‘If what the sensors saw last December was so important that the president had to call and say, “Knock it off,” why didn’t the president issue the same warning three days before the gas attack in August?’
Hersh also popped up in the headlines a couple of months ago alleging a major coverup surrounding Bin Laden’s death, if you remember, but didn’t reveal details about that story (it’s part of a book he’s currently writing).