WASHINGTON (AFP) - General Joseph "Fighting Joe" Dunford became the US military's highest-ranking officer on Friday, chosen by President Barack Obama as America's armed forces face an array of global crises and combat challenges.
Dunford, 59, replaces General Martin Dempsey as chairman of the Joint Chiefs of Staff.
Though the position has no direct operational authority, he acts as the main military adviser to the President and the defence secretary, and holds massive sway on matters of war and peace.
"Fighting Joe," who earned his nickname as an infantry officer during the US-led invasion of Iraq in 2003, arrives on top of the military food chain during a difficult time.
Recent years have seen a resurgent Russia annex Crimea and aid separatists in eastern Ukraine, as well as the swift rise of the Islamic State group in Iraq and Syria that took the world by surprise.
In addition, the military is facing cuts if the Republicans and the White House can't agree on the federal budget.
Observers say Obama picked Dunford because of his ability to give candid and impartial advice.
"He doesn't fabricate things, he doesn't play to an audience, but he also doesn't get upset. He's level-headed, calm and direct," Rand Corporation senior analyst Ben Connable told AFP.
"He knows how to tell you your baby's ugly without making you upset."
Dunford officially starts Oct 1, but was sworn in during Dempsey's colourful retirement ceremony in Virginia on Friday, which Obama attended and which saw the outgoing general sing an Irish folk song.
Dempsey, 63, served nearly four years in the top general's spot, but drew criticism from opponents who said he should have advocated a stronger stance against Islamic State in Iraq and Syria (ISIS) militants.
Drawing on the experience of the US-led war in Iraq, which lasted almost a decade, he was against sending troops back to the Middle East to fight ISIS, and instead wanted local Iraqi forces to do more fighting.
It was a policy critics said emboldened America's enemies.
During his time as commander in Kabul, Dunford oversaw a major drawdown of tens of thousands of US troops, as Afghan forces took the lead in the fight against the Taleban insurgency.
But Dunford persuaded the White House to keep a small force on the ground after 2014 instead of a total troop withdrawal.
Dunford also led troops in the 2003 US-led invasion of Iraq and spent nearly two years in the country.
In 2009, he was appointed head of Marine forces in the military's Central Command, which oversees American troops across the Middle East.
An infantry officer, Dunford followed General James Amos, a pilot, as commandant of the Marine Corps.