"Sudden ally" refers to someone who has suddenly become an ally or supporter. There are a variety of synonyms for this phrase, including "unexpected ally," "surprise ally," and "accidental ally." These terms describe someone who was not expected or planned to become an ally, but who has decided to offer their support nonetheless. Other synonyms for "sudden ally" include "unintentional ally," "unforeseen ally," and "unsuspected ally." These words convey the sense that the ally in question has arrived on the scene unexpectedly, but is nevertheless a valuable addition to one's support network.