There are a variety of different synonyms for the phrase "was one obligation." Some commonly used alternatives might include "was a responsibility," "was a duty," "was a requirement," or "was a mandate." Other potential synonyms might include "was a commitment," "was a necessary task," or "was an essential obligation." The specific synonym that an individual might choose to use will depend on the context in which the phrase is being used, as well as the tone and style that the writer is trying to achieve. Ultimately, the goal is to find a synonym that accurately conveys the meaning of the original phrase while also fitting within the overall flow and structure of the sentence or paragraph.