Donald Trump's pledge to block Nippon Steel's planned purchase of U.S. Steel if he retakes the White House compounds political troubles for the $15 billion deal whose failure could reverberate across Japan Inc. For Japan, the biggest foreign investor in the U.S., a collapse of the deal could give companies pause about acquisitions in other strategic sectors and force them to be more risk averse when sizing up deals, say former officials, lawyers, analysts and executives. The planned purchase by the world's fourth-largest steelmaker of the storied American firm underlines the limits of "friendshoring", a term coined by U.S. officials to describe deeper economic cooperation between allies, an effort partly aimed at de-risking supply chains from rival power China.