Final answer:
The United Kingdom left the European Union on January 31, 2020, after a referendum in which a majority voted for Brexit.
Step-by-step explanation:
The country that left the European Union on January 31, 2020, was the United Kingdom (UK). The decision to leave, known as Brexit, followed a 2016 referendum in which 52 percent of UK citizens voted in favor of leaving the EU. This historical event concluded the UK's membership within the EU's trading bloc.
Supporters of Brexit had argued that the EU was economically dysfunctional and infringed on British sovereignty, while opponents feared the adverse economic impacts, such as potential trade disruptions and higher costs for goods.
Despite various challenges, including the differing votes from Scotland and Northern Ireland and complexities concerning the border between Ireland and Northern Ireland, the UK's withdrawal was completed in January 2020, following a withdrawal process triggered in March 2017.