At war with is a phrase that refers to a state of conflict between two or more individuals, groups, or countries. There are a number of synonyms that you can use instead of this phrase to convey the same meaning, such as in conflict with, engaged in combat with, at odds with, battling against, fighting with, and in opposition to. Additionally, other synonyms could include contending with, struggling with, warring against, clashing with, and grappling with. These synonyms can be interchanged in order to add variety and nuance to your writing, and can help to create a more dynamic and engaging text.