Chemical weapons attacks are notoriously difficult to substantiate. The debris they leave is hard to detect from satellite images. To positively identify a toxic agent, you need to analyze affected blood samples. But in Syria, and in particular in the Damascus suburb where the December attack occurred, the specialized medical equipment needed to conduct these tests is in severely short supply.
The removal from Syria of the Assad regime's stockpile of chemical weapons shows that joint efforts can yield positive results. Likewise, by agreeing to extend the international negotiations on Iran's nuclear program, the parties to the talks have kept alive the promise of a final deal, which would be a great victory for multilateral diplomacy.