Heres two examples for the US:
1. US Civil War (American Civil War) 1861-1865
2. Vietnam War 1961-1975.
The US Civil War (American Civil War) was an undeclared war and had many naval battles; are you asking about that war?
No, it was a declared by both belligerents.
Be more specific in your question
in the Atlantic ocean.
the XYZ affair
US Civil War & Vietnam War are two examples.
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)
The definition of the word undeclared is something that has yet to be declared or announced to the public. For example a businesses accounts may remain undeclared for a time.
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.
It was an undeclared war.
No, it was a declared by both belligerents.