US President-elect Donald Trump has named General James Mattis as his defence secretary.
Trump has previously described Gen Mattis, 66, as "a true general's general".
Gen Mattis, who is known as "Mad Dog", was an outspoken critic of the Obama administration's Middle East policy, particularly on Iran.
He has referred to Iran as "the single most enduring threat to stability and peace in the Middle East".
Trump made the announcement in Ohio at the start of a "USA Thank You Tour 2016" for his supporters.
Gen Mattis is a former marine with battlefield experience.
He led an assault battalion during the first Gulf war in 1991 and commanded a task force into southern Afghanistan in 2001.
He also took part in the invasion of Iraq in 2003 and played a key role a year later in the Battle of Fallujah.
