Sentence:  But all of us took an <WSD>oath</WSD> to do our duty when we joined the Space Force, and I fully expect everyone to willingly keep their word. But you took no <WSD>oath</WSD>, and have no obligation. 
Meaning of the word marked with <WSD> tags:  A solemn pledge or promise, appealing to a deity, a ruler, or another entity (not necessarily present) to attest to the truth of a statement or sincerity of one's desire to fulfill a contract or promise.