Marine Gen. James Mattis is something of a legend in the US military. Looked at as a warrior among Marines, and well-respected by members of other services, he's been at the forefront of a number of engagements.
He led his battalion of Marines in the assault during the first Gulf war in 1991, and commanded the task force charging into Afghanistan in 2001. In 2003, as a Major General, he once again took up the task of motivating his young Marines to go into battle.
One day before beginning the assault into Iraq, on March 19, 2003, every member of 1st Marine Division received this letter, written in Mattis' own hand.
In the letter, he tells them, "on your young shoulders rest the hopes of mankind." He conveys a sense of staying together and working as a team, writing, "keep faith in your comrades on your left and right and Marine Air overhead. Fight with a happy heart and a strong spirit."
He finally signs off with the motto of 1st Marines: "No Better Friend, No Worse Enemy."
You can see the full letter below: