Which term best describes a nation that allows allied forces to operate within its territory?

The term "Host Nation" best describes a country that permits allied forces to operate within its borders. This arrangement signifies cooperation and support, often involving agreements that outline the terms and limitations of the allied forces' presence. A host nation benefits from the protection, resources, and military support provided by the allied forces and may also foster closer diplomatic and security ties as a result.

In contrast, the other terms do not fit the definition adequately. An adversary refers to a nation that opposes or is in conflict with another state, which would not align with the concept of allowing allied forces to operate. A partner nation generally indicates a country that collaborates with others but does not inherently imply that it hosts foreign military forces. A neutral party describes a state that does not take sides in conflicts and typically would not permit foreign military operations, as doing so could compromise its neutral status.