The planned 50% tariff on copper imports may turn out to be the biggest own goal of U.S. President Donald Trump's ongoing trade war with the rest of the world.

President Donald Trump sent out tariff letters to six smaller U.S. trading partners on Wednesday with a pledge to announce import taxes on other countries later in the day.

Oil prices dropped on Thursday as the latest tariff announcements by U.S. President Donald Trump were perceived by market participants to threaten global economic growth and demand for the resource.