Yes, they happen all the time.
Both were undeclared wars.
The North won and the South lost in both undeclared wars; US Civil War & Vietnam.
American Civil War & Vietnam War.
The opium wars were never signed. Wars are declared or undeclared, not signed.
Undeclared wars
We have engaged in several undeclared wars, such as the undeclared Naval War with France, under President Adams, and the War with the Barbary Pirates under Jefferson. Some of the conflicts, like Korea, were termed Police Actions, not wars. However, Congress did give approval for the use of US troops. Some Indian battles in the west were carried out without a formal declaration against that particular tribe or settlement. Every military action since the end of WWII have been undeclared wars. Korea, Vietnam, Gulf War, Iraq..etc....
American Civil War 1861-1865. Vietnam War 1961-1975.
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.
They were in conflict over political ideology which resulted in the Cold War and a variety of undeclared wars which were fought primarily by their surrogates.
Undeclared wars: US Civil War, Vietnam War. Declared wars: War of 1812; Mexican War of 1846-1848; Spanish-American War of 1898; WWI; WWII. Just a few examples.
The duration of Undeclared is 1320.0 seconds.
Two examples: 1. American Civil War 1861-1865 2. Vietnam War 1961-1975