The US Army motto, signifying their willingness to execute their duties as soldiers within the US Armed Forces; This refers to the US, the Constitution, and its constituents. We'll refers to the Army's collective ideal of teamwork within soldiers. Defend refers to the Army's main task - being to defend the United States from enemies, both foreign and domestic, as well as support and defend the Constitution.
NCOiC: "This we'll defend."
DS: "'Til the end."