From the introduction for US Decision to Cease Implementing the Iran Nuclear Agreement (R44942 May 9, 2018):
On May 8, 2018, President Donald Trump announced that his Administration would cease implementing U.S. commitments under the 2015 multilateral Joint Comprehensive Plan of Action (JCPOA) with Iran and reimpose all U.S. sanctions that were in place prior to the JCPOA. His announcement made reference to his previous statements on the issue, including an October 13, 2017 announcement of U.S. strategy on Iran and a January 12, 2018, statement pledging to leave the agreement unless Congress and U.S. allies acted to address the full range of U.S. concerns on Iran. In his May 8 and earlier statements, President Trump asserted that the agreement does not address the full range of potential threats posed by Iran, or permanently ensure that Iran cannot develop a nuclear weapon. President Trump also again asserted that asserted that the JCPOA provided Iran with additional financial resources with which to pursue its ballistic missile program and support its regional “malign activities.” In his May 8 statement, President Trump indicated that reimposed U.S. sanctions would succeed in pressuring Iran to renegotiate the JCPOA to accommodate Administration demands.