From the Summary: "Successive Administrations have used sanctions extensively to try to change Iran's behavior. Sanctions have had a substantial effect on Iran's economy but little, if any, observable effect on Iran's conventional defense programs or regional malign activities. During 2012-2015, when the global community was relatively united in pressuring Iran, Iran's economy shrank as its crude oil exports fell by more than 50%, and Iran had limited ability to utilize its $120 billion in assets held abroad. [...] On April 22, 2019, the Administration announced it would not renew any SREs [Significant Reduction Exceptions] when they expire on May 2, 2019, instead seeking to drive Iran's oil exports as close to zero as possible. On May 3, 2019, the Administration ended some waivers for foreign governments to provide technical assistance to some JCPOA [Joint Comprehensive Plan of Action]-permitted aspects of Iran's nuclear program. The economic difficulties and other U.S. pressure measures have prompted Iran to cease performing some of the nuclear commitments of the JCPOA."
CRS Report for Congress, RS20871
Congressional Research Service: https://crsreports.congress.gov/