A partisan battle over President Trump’s efforts to divert Defense Department money to build a wall on the U.S.-Mexico border is poised to snarl House and Senate negotiations over upcoming defense policy and spending bills, despite lawmakers having already reached a broad budget agreement. “It is a highly political issue that could very well derail the [defense authorization bill] conference,” said one congressional staffer. At issue is $6.1 billion in Pentagon funding the White House has ordered diverted to pay...