"This we'll defend" is the official motto of the United States Army. It refers to the army's primary purpose of defending the country as one. It dates back to before the American Revolutionary War.
When the War Office was created in 1776 to manage the Continental Army, it adopted "This we'll defend" as its official motto. Each word is meaningful. "This" refers to the United States, the U. S. Constitution and liberty. "We'll" references the army's collective team efforts, and "defend" refers to the primary function of the army as a defender of the nation, not an aggressor against others.
In the official U.S. Army logo, "This We'll Defend" is found in a rattlesnake-encircled banner above a tableau filled with military and revolutionary symbols, including a liberty cap, cannons and a Roman cuirass.