WASHINGTON-President-elect Donald Trump has tapped retired Marine Corps Gen. James Mattis to become secretary of defense-a choice that requires Congress to pass a special law for the recently retired military officer to take up the Pentagon's top post.
"We are going to appoint 'Mad Dog' Mattis as our secretary of defense," Mr. Trump said at a rally in Cincinnati on Thursday, referring to the general by a nickname. Mr. Trump's speech in Ohio was the first stop in a nationwide "Thank You" tour he will conduct over the next few weeks.
The decision brings a longtime military hand, known for his battlefield toughness and intellectual heft, to the lead of the nation's armed forces as thousands of U.S. troops remain on the ground in Afghanistan and Iraq. Gen. Mattis rose through the ranks of the Marine Corps over 43 years to become the U.S. military's top commander in the Middle East.
Since retiring in 2013, Gen. Mattis has become a vocal critic of what he calls Washington's "strategy-free" approach to warfare and threats. He has urged the U.S. to engage in the Middle East more directly and without preset limits on the use of force, echoing critics of President Barack Obama's administration who say it has taken an overly incremental and fair-weather approach.
He has agreed with Mr. Trump about taking more intensive action against U.S. enemies such as Islamic State but also has articulated views that contradict Mr. Trump's campaign-trail talking points, setting up potential battles over policy.
Gen. Mattis serves as a director at embattled blood-testing startup Theranos Inc. and is on the board of General Dynamics Corp., the defense contractor and aerospace firm that builds nuclear-powered submarines, armored vehicles and business jets.
The selection of Gen. Mattis adds to Mr. Trump's reliance on former military officers for his national-security team. The president-elect chose retired U.S. Army Lt. Gen. Michael Flynn as his national security adviser and is considering Navy Adm. Michael Rogers to become director of national intelligence. He has also spoken to another retired Marine Corps general, John Kelly, about a top administration post.
Most previous secretaries of defense served in the military in some capacity, the majority of them in their youth. U.S. law, however, requires a veteran to be out of uniform for seven years to qualify for the civilian post. Gen. Mattis retired 3½ years ago, meaning Congress will need to pass a special law giving Mr. Trump permission to nominate him.
The only other recently retired general to become secretary of defense since the position's creation after World War II was George C. Marshall. Congress passed a law to permit his appointment.
Gen. Mattis enjoys widespread popularity on Capitol Hill, where many top lawmakers say they would vote to approve a law to allow for his appointment, but the process will force Congress to address broader questions about civilian control over the military. When Congress passed legislation permitting Gen. Marshall's appointment in 1950, the law expressly said such a move wasn't to be repeated.
One Democratic senator said she would oppose a law allowing for Gen. Mattis's appointment.
"While I deeply respect Gen. Mattis's service, I will oppose a waiver," said Sen. Kirsten Gillibrand of New York, the senior Democrat on the Senate Armed Services Committee. "Civilian control of our military is a fundamental principle of American democracy, and I will not vote for an exception to this rule."
However, Robert Gates, who served as a defense secretary under President Barack Obama as well as former President George W. Bush, defended such an action for Gen. Mattis. Mr. Gates said Gen. Mattis would provide stability for uniformed Americans.
"Normally, I would be concerned about civil-military relationships by having a former senior officer, particularly one so recently in uniform, in that job," Mr. Gates said in an interview with The Wall Street Journal. "But Gen. Mattis is so deeply steeped in history and is such a strategic thinker and brings such extraordinary experience to the table, that I think that this would be one time that's worth making an exception."
Gordon Lubold and Julian E. Barnes contributed to this article.
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