Trump has appointed General H.R. McMaster as his new national security adviser. The H.R. stands for Herbert Raymond, but the West Point grad with a PhD in American history just goes by H.R.
McMaster served with Secretary of Defense James Mattis in Iraq, and is now a three star General. He'll remain in the military while serving as Trump's national security adviser.
He wrote a popular book on the Vietnam War — Dereliction of Duty — which blamed the then-Joint Chiefs of Staff for not standing up to President Johnson and preventing the war. McMaster is known for being outspoken, and was listed as one of the 100 most influential people by Time Magazine in 2014.
How he differs from Trump: Compared to predecessor Michael Flynn, McMaster is much more moderate on his stance toward Islam, being careful to separate the religion from extremist groups. He also considers Russia a growing threat, according to Politico, and has focused on how to fight their tactics.