There are a variety of synonyms for the phrase "become allied with." Depending on the specific context and tone, some alternatives might include "join forces with," "align oneself with," "affiliate with," or "form a partnership with." Additionally, one might consider using phrases like "cooperate with," "strategically align with," or "collaborate with." The choice of synonym may depend on the specific nuances and implications that the writer is trying to convey. For example, "alliance" might suggest a temporary agreement or a lack of deep commitment, while "partnership" could imply a more long-term and intertwined relationship.