President Donald Trump on Monday signed an executive order to secure U.S. defense supply chains, including the critical materials necessary for weapons manufacturing. The order makes it harder for U.S. defense contractors to obtain a waiver that would allow them to buy minerals from prohibited foreign suppliers like China. The order means the contractors will have to do a lot more to prove that a purchase from a prohibited supplier is necessary than they have in the past. “No more: ‘we tried nothing and we’re out of options,’” White House adviser Peter Navarro said on a briefing call with reporters….