1. US law provides for ratified treaties to have the same force as federal law (https://en.wikipedia.org/wiki/Treaty_Clause)
2. The US and Iran have both ratified the NPT
3. The NPT guarantees the rights of signatories to establish peaceful nuclear programs
4. It's therefore, in theory, illegal under US federal law (which of course, at least applies to US persons) to interfere with the establishment of a peaceful Iranian nuclear program.
The practice of course, differs greatly from the theory. This is because, as per usual, law is more about politics than law.
1. US law provides for ratified treaties to have the same force as federal law (https://en.wikipedia.org/wiki/Treaty_Clause) 2. The US and Iran have both ratified the NPT 3. The NPT guarantees the rights of signatories to establish peaceful nuclear programs 4. It's therefore, in theory, illegal under US federal law (which of course, at least applies to US persons) to interfere with the establishment of a peaceful Iranian nuclear program.
The practice of course, differs greatly from the theory. This is because, as per usual, law is more about politics than law.