With the start of an Iranian nuclear facility, (that is enriching uranium) it seriously calls into question the credibility of the UN and the US itself.
The UNSC has made numerous demands that Iran "halt all enrichment." Not high-level enrichment, or enrichment at a certain level, but all enrichment activities. Iran said no, and in response there are no consequences.
Coming on the heels of Iraq, and now Iran, there is a very good argument that the United Nations has lost all credibility on the world stage in terms of conflict scenarios.
The United States on the other hand has openly stated that an Iranian nuclear weapon would be unacceptable, and this nuclear facility is a major step in that direction. While its existence does not automatically mean Iran will obtain a weapon, it does mean that Iran can obtain one in a much shorter time frame, and possibly without the knowledge of the US.
Either way, the United States looks ridiculous arguing that Iran must not be allowed to obtain a nuclear weapon, and when they make a major step in that direction, we call it "not a threat."
In international relations, credibility is everything, and this event is a blow to both the United States and certainly a major one to the United Nations.