An "un-declared war" is a war in which the combatant nations (the countries fighting each other) do not write papers (complete paper work formalizing the war) stating that they are at war with each.
Another words: Its like a reciept when buy something, people want to see IT IN WRITING. War is seeing IT IN WRITING.
Two examples of US undeclared wars are the US Civil War and Vietnam War.
The US Civil War (American Civil War) was an undeclared war and had many naval battles; are you asking about that war?
The cast of The Undeclared War - 1966 includes: Ted Yates as himself
Quasi-War
Undeclared War - 2013 is rated/received certificates of: Greece:K-13 (video rating)
North Vietnam.
The North won and the South lost in both undeclared wars; US Civil War & Vietnam.
The Quasi-War was an undeclared war fought almost entirely at sea between the United States and France from 1798 to 1800. In the United States, the conflict was sometimes referred to as the Franco-American War, the Undeclared War with France, the Pirate Wars, or the Half-War.
American Civil War & Vietnam War.
US Civil War & Vietnam War are two examples.
It was an undeclared war.
Be more specific in your question
No, it was a declared by both belligerents.