Despite slowing over the first half of the year, the U.S. economy has remained reasonably healthy. Yet when the jobs report for July was released on Friday, showing a substantial slowdown in hiring, President Trump lashed out, claiming the figures were rigged and firing the head of the government agency that produces them.

Dmitri Medvedev was once president of Russia but now is little more than the Kremlin’s favorite online troll. Yet when he got under Mr. Trump’s skin with provocative posts about nuclear war, Mr. Trump, already increasingly infuriated by President Vladimir V. Putin’s unwillingness to work with him to end the war in Ukraine, responded on Friday as if a real superpower conflict could be brewing, ordering submarines into position to guard against any threat.

Just days earlier, Mr. Trump had returned to the United States from a golf trip happily flexing his political and diplomatic power.

A capitulating Congress had passed his signature domestic policy legislation, despite concerns over its deep cuts to the social safety net. The European Union caved to Mr. Trump and his threat of tariffs by announcing a trade deal during the president’s trip to Scotland. Emboldened, Mr. Trump moved ahead with sweeping tariffs that could reshape the world economy.