When U.S. President-elect Donald Trump assumes office in January, an array of vexing foreign policy and security challenges will await him. The next administration will have its hands full between the conflicts in the Middle East, the enduring standoff with Russia, the competing claimants in the South China Sea and the upheaval in the European Union. On top of these thorny issues, Trump will have to contend with matters of nuclear weapons proliferation, balance and arms control, pressing concerns that are sure to command the next president's attention....