To understand the issue, you must first understand what led to the sanctions.
When Iran first sought to develop atomic energy, they had to agree to the conditions set out by the IAEA, to get access to nuclear technology maintained by other nations who develop nuclear power.
This includes agreeing not to use that technology to develop nuclear weapons, the Non Proliferation Agreement. Meeting that obligation means allowing random inspection of all facilities that deal with nuclear technology.
Iran got in trouble, and got sanctions, when it expelled the inspectors and refused to allow inspections.
In order to get the sanctions lifted, and all IAEA nations had to agree, not just Obama, Iran agreed to the resumption of inspections. Theoretically, if Iran lives up to that agreement, it will not develop nuclear weapons.
If it does develop nuclear weapons, that will be a clear violation of the agreements.
Is Iran developing nuclear weapons? At this point, that is a matter of opinion, though expelling the inspectors would have been the first step... the first step in N Korea developing nuclear weapons was to expel the inspectors.
One does have to wonder why a nation awash in oil feels it necessary to develop the very expensive option of generating electrical power via nuclear reactors.