President Trump fired his secretary of state, Rex W. Tillerson, with a tweet this week.

Experts, and news reports, have emphasized the unusual nature of Mr. Trump’s relationship with a secretary who reportedly called him a moron — and never denied it.

But Mr. Tillerson’s difficulties during his unusually short term also illustrate how the job has changed in recent years. We spoke to several experts and looked at past coverage to explore here how the role was transformed after World War II.

Image Thomas Jefferson became the country’s first Secretary of State in March of 1790. Credit... Hulton Archive/Getty Images

The most powerful role in the cabinet?

The secretary of state, fourth in line to the presidency, was once considered the most powerful cabinet post. With four exceptions, they have always been white men, starting with Thomas Jefferson, the first to hold the position, in 1790.