In the primary debate last night, Trump said he would order soldiers to break the law. That had specific reference to waterboarding (or worse) and targeting the families of terrorists.
It should go without saying that a President has no legal authority to order subordinates to break the law. Not only are subordinates not obliged to obey unlawful orders, but they are obliged to disobey unlawful orders.
In addition, subordinates can get into legal hot water if they obey unlawful orders. It is morally as well as legally wrong for a President to put his subordinates in legal jeopardy. And we have examples in American political history of subordinates who become fall guys for Presidential illegalities.
If a particular tactic is currently illegal, and a President deems that tactic to be necessary to combat terrorism, then he should negotiate with Congress to change the law.