An era of free trade ends, as President Donald Trump slaps tariffs on imported goods. What does the varied history of trade policy suggest we should expect?

The S&P 500 (SNPINDEX: ^GSPC) has fallen 11% from its record high as of April 3, erasing over $5 trillion from the U.S. stock market. Investors have become increasingly unsettled by the abrupt and radical shift in U.