Question

Name two undeclared wars

Answer 1

The Vietnam war and U.S Civil War

Answer 2

I can name more than two for you, if we are talking about the U.S.A.:
(Take note, that I listed wars that took place after the 70s, if I seem to remember)
First Gulf of Sidra Incident 
Lebanese Civil War 
The bombing of Libya 
Iran-Iraq War 
Invasion of Panama 
First Gulf War
Somali Civil War 
Bosnian War 
Operation Uphold Democracy
Bombings of Afghanistan and Sudan 
Kosovo War
War on Terror
Second Liberian War
Lord's Resistance Army Insurgency
Libyan Civil War