The economic warning signs are flashing yellow. In Germany, the U.K., Italy, Brazil and Mexico, growth has stalled. In Japan, business confidence has turned negative for the first time since 2013. China’s industrial output is growing at its slowest pace in 17 years. An inverted yield curve has U.S. economists on recession watch. But the real uncertainty is coming from the biggest surge in decades of the kind of political volatility that is relevant to markets.