June 23, 2016 — the date of the Brexit referendum — remains a seismic moment in the UK’s political history. But the roots of the country’s long period of political instability, which claimed its latest prime ministerial victim on Monday, run even deeper. Keir Starmer’s resignation means he will, remarkably, become the fourth prime minister to leave office in the last four years. His successor will become the seventh premier since the 2016 referendum after David Cameron, Theresa May, Boris Johnson, Liz Truss, Rishi Sunak and Starmer.

Kier Starmer's resignation means the U.K. will welcome its seventh prime minister in a decade when his successor is chosen.

When British Prime Minister Keir Starmer said Monday he is stepping down, he became the sixth person to stand in front of No. 10 Downing Street to make a farewell speech in a…