The ambiguity is resolved by native English speakers by turning fight into a verb and then declaring who they are either fighting with or against, or sometimes both. Often the context makes it clear or clearer.
"he/she/they/the Russian army/people etc are fighting with the Poles against the vietnamese"
"Mr Smith is fighting Mrs Smith again" reasonable to assume it is a family dispute but it might not be.
"Mr Smith is fighting with Mrs Smith" from the context it is reasonable to assume it is possibly a family dispute but it may not be
"Mr and Mrs Smith are fighting together In the army" which army and if they are actually together in the same unit or war zone we do not know.
"Mr Jones has gone of to fight with the Belgian Army" ambiguous
"Mr jones has gone of to fight IN the Belgian Army" is how we specify as part of an army or group.
"Mr Daniels is Fighting the neighbours"
"started a fight with" at least in BR English the word started would be understood as meaning began to fight against somebody or a group of people.