When Lieutenant General HR McMaster enters the West Wing of the White House as the US national security adviser, the widely respected soldier and military historian will face one of the biggest challenges of his career.

President Donald Trump on Monday named Gen McMaster — a decorated three-star army general who commanded US troops in Iraq and also served in Afghanistan — to succeed Michael Flynn, who was fired after 24 days in the role. Mr Trump first asked Robert Harward, a retired admiral, to succeed Mr Flynn, but he offered the job to Gen McMaster after Mr Harward turned him down over concerns about lines of authority and staffing inside the White House.

Gen McMaster is highly regarded inside the military. During the 1991 Gulf war, his troops destroyed a large number of Iraqi tanks in a battle that developed his reputation as a smart tactician. Over the past two decades, his peers have come to see him as a brilliant strategist — a reputation cemented by his role creating and implementing counter-insurgency operations during the second Iraq war.