Iran is different from Syria. It is important that the distinction between the two contexts be grasped accurately if the West is to be successful in both countries.
The world, including the 5 permanent members of the United Nations Security Council, is bound by treaties banning chemical and biological weapons making any claim by Syria to them tenuous at best. Syria is legally bound by the Geneva Protocol of 1925 after World War I not to use chemical and biological weapons in war but is not a signatory to the global chemical and biological weapons ban, thus making the threat of the use of force against Syria in chemical and biological weapons diplomacy workable to bring Syria completely into the fold of the global chemical and biological weapons bans.
The threat of the use of force backing up nuclear diplomacy will not work with Iran. A treaty banning nuclear weapons does not yet exist thus removing any legal ground for the West in its negotiations with Iran. The only country to have used nuclear weapons in war is the United States, a fact which further weakens any standing the US may have unless it commits to a timeline of total nuclear disarmament similar to it relinquishing chemical and biological weapons as a member of the global conventions on chemical and biological weapons. More importantly, Iran has always maintained that its nuclear program is for peaceful purposes.
Legally Iran is on a firmer footing, even if, akin to India, it develops nuclear weapons indigenously. In Iran’s favor, unlike India, Iran is a signatory of the nuclear non-proliferation treaty (NPT), though indigenous production of nuclear weapons can be interpreted as proliferation of nuclear weapons in violation of the treaty by all nuclear powers.
It is imperative that the West understands every country context for what it is instead of muddling matters of war and peace.