When it comes to the phrase "open hostilities", there are a variety of synonyms that can be used to convey the same meaning. One such phrase might be "full-scale battle", which suggests a high level of intensity and engagement between opposing forces. "Combat" is another synonym that can be used to describe a similar situation, emphasizing the physical confrontation between opposing parties. Other possibilities might include "warfare", "armed conflict", or "conflict zone", depending on the context in which the term is being used. Ultimately, the key is to choose a synonym that accurately reflects the gravity and nature of the conflict at hand.