The President absolutely has that power. The US Supreme Court in ex parte Quirin approved the use of military tribunals, but also ruled that they were not necessary.
The tribunal proceedings in 1942 were not a trial of any sort. They were not conducted according to the rules for courts martial.
The sole function of the tribunal was to affirm that the individuals to be executed were in fact Germans out of uniform and behind our lines.
In the ZM case, such fact finding was unnecessary.
George W. Bush could absolutely have ordered ZM's execution at any time after 9:03 AM on 9/11/01, solely on his own authority.
That he did not do so is shameful.
I believe a President's role should be one of delegation. Thatmeaning hemakes decisions about how to move forward and then others carry it out. Saddam ordered executions, I do not want that power in the hands of one man, several people yes but not one person.