How will President-elect Donald Trump's pick for defense secretary, retired Marine general James Mattis, steer America's largest bureaucracy and the world's biggest war machine through what likely will be turbulent years to come?

Here are six of the most pressing issues that will land on Mattis's plate if he is confirmed:

- Iran -

Mattis is hawkish on Iran and has publicly called it the "single most belligerent actor in the Middle East."

The former Marine general, who commanded a division during the invasion of Iraq, has blamed the deaths of some US troops on Iranian support for Shiite militias in Anbar province.

And Mattis has accused Tehran of continuing to finance extremist networks that are destabilizing the Middle East.

Trump has slammed the Iran nuclear deal, and Mattis has also challenged it.

He has said the White House isn't doing enough to counter Iranian military moves in the region -- and is likely to push for a hard line against the Islamic Republic.

- Defeating the Islamic State group -

During the campaign, Trump said he would "bomb the shit" out of the IS group, pledged to kill the relatives of suspected terrorists and to torture captives.

Mattis, however, has told Trump that he doesn't agree and the president-elect has seemed to cool on his torture pledge.

Mattis may struggle to come up with an anti-IS plan that diverges massively from the one already being pursued by the Obama administration -- namely, to bomb IS targets and train and equip local forces to kill the jihadists.

We will "pursue aggressive joint and coalition military operations to crush and destroy" IS, Trump said in his foreign policy statement.

Trump last year suggested sending US troops into the region to snatch oil fields, but his public plan does not mention this.

- Military spending boom? -

The US military is by far the world's most powerful and most expensive -- with bases spanning the globe, an annual budget of more than $600 billion and about 1.3 million active-duty troops.

China is the world's second biggest defense spender, but with an annual budget that is about a quarter the size.

Trump said Thursday that his administration "will begin a major national effort to rebuild our badly depleted military." He wants more ships, troops, planes and weaponry.

Mattis would oversee a surge in active troop numbers, with the Army growing to 540,000 under Trump's pledges (up from the current planned number of 450,000) and an increase of ships and submarines to 350 (up from 308).

Trump also wants more warplanes and a "state of the art" missile defense system. US arms firms have seen stock prices surge amid a broader market rally.

But Trump's plans to boost spending would require Congressional approval.

- Demanding more from allies -

During his campaign, Trump accused NATO members and Asian allies of not paying their fair share to long-standing alliances underpinning regional security.

But since his election, Trump has dialed back some of this rhetoric.

A first order of business for Mattis would be to clarify with allies just what the new administration's position is.

The United States maintains hundreds of bases and military sites around the world and currently has more than 200,000 military personnel deployed across dozens of nations, including 28,500 in South Korea and about 50,000 in Japan.

- Afghanistan -

Fifteen years and hundreds of billions of dollars since the US-led invasion of Afghanistan in 2001, the security situation in the country remains fraught and Afghan security forces are struggling to contain a resurgent Taliban.

President Barack Obama was forced to slow a planned withdrawal of US troops, and about 8,400 will remain in the country when he leaves office.

Afghanistan got scarcely a mention during the campaign, but Trump on Thursday indicated a reluctance to intervene overseas.

"We will stop looking to topple regimes and overthrow governments, folks," he said. "Our goal is stability, not chaos."

Mattis, who has led troops in Afghanistan, had criticized Obama's plan to pull forces from the country.

- Russia -

Trump has openly praised Russian President Vladimir Putin's leadership abilities and was accused by his Democratic rival Hillary Clinton of being a "puppet" for America's longtime foe.

Russia has seized Crimea, carried out military exercises on its border with Eastern Europe and has for more than a year conducted an intense bombing campaign in Syria to prop up President Bashar al-Assad.

It remains to be seen whether Trump could ask Mattis to find ways to better coordinate with Russia in Syria or elsewhere, an idea once unthinkable for the US military.