President-elect Donald Trump's tweets are causing a firestorm among defense stocks.First, Trump blasted Boeing Co over the soaring cost of manufacturing the next-generation Air Force One fleet. The president-elect also targeted Lockheed Martin Corporation on December 12 over the rising cost of the F-35 program, and then followed up on Thursday with a tweet suggesting the company's F-18 contract could be handed over to Boeing due to the "tremendous cost and cost overruns."So, how the heck are investors supposed to invest in defensive stocks when a single tweet from Trump results in a selloff?Doug Ramsey, The Leuthold Group chief investment officer, was a guest on CNBC to answer this pressing question.Read more