In a nutshell, Former SOS John Kerry, (who served in Vietnam) says he was "surprised" President Obama did not go ahead with military strikes in Syria, admitting the U.S. "paid a price" for not enforcing a his red line over the use of chemical weapons.
Kerry says he wanted to take more hard line approach to Assad and Syria, but Obama didn't want it. He tells CBS News he 'made it clear' to Obama his position on Syria from the 'moment he was nominated.' Kerry explains that he believes O was worried about 'where Syria would go' if he did what Kerry proposed. John Kerry even say he partially 'supported' President Trump's actions on Syria.
BTW, there have been a few recent headlines saying that Kerry is open to a 2020 run, this interview would lead one to believe those headlines are true.