President Trump reignited his trade war with Europe at a particularly bad time for the stock market.The S&P 500 (^GSPC 2.06%) added 14% during the past year despite President Trump's tariffs hiking the average tax on U.S. imports to its highest level since 1935. However, while the stock market's strong performance is encouraging, investors have several reasons to be nervous:Trump claimed exporters would absorb the tariffs, but Goldman Sachs research indicates that U.S. companies and consumers collectively p ...