Why did they wait for the horse to bolt?

Our all-seeing all-knowing government has published the 'legal advice' it used to bolster its case for bombing Syria. The advice, which could almost fit on the back of a fag packet (were it not for the health warnings) concludes:

In these circumstances, and as an exceptional measure on grounds of overwhelming humanitarian necessity, military intervention to strike carefully considered, specifically identified targets in order effectively to alleviate humanitarian distress by degrading the Syrian regime’s chemical weapons capability and deterring further chemical weapons attacks was necessary and proportionate and therefore legally justifiable. Such an intervention was directed exclusively to averting a humanitarian catastrophe caused by the Syrian regime’s use of chemical weapons, and the action was the minimum judged necessary for that purpose.

This rather begs the question why they didn't unleash their highly accurate, GPS guided missiles BEFORE the most recent chemical weapons use in Syria. It appears as if it took no time at all for them to identify "carefully considered, specifically identified targets."  So for how long had May's government known of these targets, and why didn't they 'take them out' before yet another "humanitarian catastrophe" happened? Isn't prevention better than cure? In her own terms our Prime Minister has demonstrated her capacity for negligence yet again.