There is no documentation in surviving records from the Revolutionary War that stipulates the configuration of the design of the War Office seal. Nonetheless, The War Office [aka Department] seal bore the inscription from 1778 to 1947. Subsequent reorganizations maintained the inscription.
The description of the Department of the Army Seal indicates this phrase has been in use since before 1775.
From the Army Institute of Heraldry regarding the symbolism of the Seal:
"This We'll Defend" on a scroll held by the rattlesnake is a symbol depicted on some American colonial flags and signifies the Army's constant readiness to defend and preserve the United States."
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