They did not act - Iran sanctions

So we acted for them From The Washington Free Beacon:

All International Sanctions on Iran Reimplemented, Killing Nuclear Deal
All international sanctions on Iran that were lifted as part of the Obama administration’s landmark nuclear deal with the country have come back into effect as of Saturday evening, removing the last remaining vestiges of the agreement that were still in place.

The renewal of these sanctions comes after months of last-ditch diplomatic efforts by the Trump administration to ensure a United Nations ban on Iran’s purchase of advanced weapons was not lifted. This effort was unsuccessful after it failed to garner backing from traditional European allies on the U.N.’s Security Council. Following that bid, the United States exercised its right under the original 2015 accord to reimpose a litany of sanctions on Tehran that have been waived since the accord was signed and endorsed by the U.N. With the reimposition of these sanctions, the nuclear deal that President Donald Trump abandoned in 2018 is effectively voided.

Members of the U.N. Security Council were powerless to stop the United States from triggering what is known as "snapback"—a mechanism written into the original agreement that permits any member nation to reimpose international sanctions. The move is likely to further enflame ongoing tensions with Iran and also complicate America’s relationship with some its closest allies, including France, Germany, and the United Kingdom. Each of these countries stood in the way of U.S. efforts to extend the Iran arms ban as they sought to keep the nuclear accord on life support.

Secretary of State Mike Pompeo blamed European powers on the Security Council for forcing the United States to invoke snapback.

The United Nations proved once again that it is ineffectual so we let a part of the Obama Iran Deal take effect. The snapback was always there - it was part of Obama's deal. We are implementing under the rules of the deal.