Stocks tumbled Friday after President Donald Trump gave the greenlight on the imposition of about $50 billion worth of tariffs on imports from China. The Trump administration has so far justified the newest round of tariffs on the basis of intellectual property theft perpetrated by China.

To think a trade war is nothing but a far off threat to U.S. markets is the dumbest thing on Wall Street this week. As one of my sources told me, we're not starting a trade war. We've been in a trade war for 20 years. Now we're moving from diplomacy to hand-to-hand combat. Salvos are being thrown, and U.S. investors better look out below.