Trump announced the nomination Thursday at his first post-election rally in Cincinnati, Ohio.

"We are going to appoint 'Mad Dog' Mattis as our secretary of defense," Trump told supporters in Cincinnati. "He's our best. They say he's the closest thing to (World War II-era) General George Patton that we have," he added.

The 66-year-old former Marine General Mattis (center in photo) retired in 2013 after being head of the US Central Command with authority over troops in Iraq and Afghanistan and responsibility for an area including Syria, Yemen and Iran.

He has been quoted as saying, "Be polite, be professional, but have a plan to kill everyone you meet."

Under current rules, Mattis would be barred from running the Pentagon until he had been out of uniform for seven years.

It would, therefore, require a vote in Congress to pass legislation to waive that rule, intended to ensure civilian control of the US military. Lawmakers have only taken a similar action once in the law's 65-year-history, for General George Marshall in 1950. He had been Chief of Staff of the US Army under Presidents Franklin D. Roosevelt and Harry S. Truman and organized the largest military expansion in US history. He retired from active service in 1945.

Policy differences

Should he be confirmed, Mattis' appointment could signal a reversal of President Barack Obama's policy on Iran. Mattis has been highly critical of it and was released from duty during the Obama administration over his disagreements with the White House over how to deal with Tehran. Mattis has described Iran as "the single most enduring threat to stability and peace in the Middle East."

It is also likely that Mattis will differ with Trump over policy towards Russia and its President Vladimir Putin. While Trump has spoken positively about working with Putin, Mattis has warned that Moscow wanted to "break NATO apart."

Earlier this year, Mattis indicated in an interview with the "Politico" website a clear difference of opinion over the NATO military alliance: "Trump's contention that US allies are not paying their 'fair share' of costs to support the alliance, [is] ‘about as kooky as [if] a president were to call our allies freeloaders.'"

On an individual level, Mattis is a great reader, with a library of 7,000 books, who encouraged the Marines under his command to read. He instructed them that the most important territory on a battlefield was the space "between your ears." He often quotes history and is proud that he grew up with a large library and no television.

jm/gsw (Reuters, AP)