Why the Trump administration is helping support Japan’s weakening yen

The United States and Japan last week staged a coordinated intervention to halt the slide of the yen after the Japanese currency fell to a 40-year low against the US dollar.

While it is unusual for authorities to intervene to help prop up another country’s currency, the yen has an important role in international finance as the world’s third-most-traded currency, meaning its depreciation has repe...