As for the Late Iranian Agreement

it always a good example "making the best of a bad situation" nothing more.  Sanctions and covert action was SLOWING not stopping Iran's path to WMDs.  Even turtles, even determined enough, will struggle past a line given enough time, non-proliferation being an ongoing policy preference of the USA we were slowly approaching a situation where either we had to militarily intervene or invoke diplomacy. Thankfully we invoked diplomacy got the program dismantled and got the IAEA back on the ground in Iran (where they damn well belong believe me).
This was a sort of a diplomatic "Hobson's Choice", did we want a nuclear armed Iran that was then making turtle speed towards reliable ICBMs or a nuclear disarmed Iran that trying to build missiles all the same.  The short term problem to me at least, is the nukes, get those off the board and then set to work on a missile regime.
All diplomatic progress is culmulative and subject to fitful evolution, but if the nuclear "agreement" was a success it stood to reason that a missile agreement could be reached as well.
Alas the nuclear agreement has been denounced by the USA (with fateful consequences for our negotiations with North Korea as well) but at the bottom of that decision lies a conflict between two world views.
A rational realist world-view that asserts interests are defined by power, that intervention holds hidden costs and that captioned actors make reasonably rational decisions. The other view, the Trumpian view is that we are manipulated abused and persecuted by our negotiating partners, we must lash back to gain respect and assert power over our hated enemies. Its a child's view of the world, but then children are rarely rational and are much given to mendacity on the rocky road to adulthood.
And that view in turn is calling the shots in DC right now...if anything it'll do nothing but spark more proliferation not less...nukes, chemical weapons, biological warfare the whole unholy menu.
Lastly, breaking the agreement fractures NATO, compels the European signatories to renegotiate (if possible) in reduced circumstances, introduces new cross cutting sanctions and worst of all hands a shit ton of soft power and leverage to Moscow and Peking.  John Bolton is talking tough now, but wait until Putin makes a State Visit to Tehran...that'll make the Whole World Gasp.