From the Document: "After the 2016 referendum in which 52% of voters in the United Kingdom (UK) favored leaving the European Union (EU), 'Brexit' was originally scheduled to occur on March 29, 2019. In early 2019, Parliament repeatedly rejected the withdrawal agreement negotiated between then-Prime Minister Theresa May's government and the EU. Boris Johnson became prime minister in July 2019, following May's resignation. Given continued political deadlock over Brexit in the UK, the EU granted the UK three deadline extensions, with the most recent lasting until January 31, 2020."
CRS Report for Congress, R45944
Congressional Research Service: https://crsreports.congress.gov/