【Enemy】 , 【foe】  denote an individual or body of individuals that is hostile or that manifests hostility to another.

【Enemy】 usually stresses antagonism that arises from a cherished hatred or a desire to harm or destroy, but it may suggest nothing much more than active or evident dislike or a habit of preying upon.

【Foe】 , on the other hand, implies active warfare.

When the reference is to a nation or group of nations with whom a country is at war,  【enemy】  is preferred in general use,  【foe】 being used in this sense chiefly in poetry or rhetorical prose.