Learner's definition of WAR
1 : a state or period of fighting between countries or groups
They fought a war over the disputed territory.
A war broke out when the colonists demanded their independence.
a nuclear war [=a war fought using nuclear weapons]
— often used before another noun
We need to resolve our conflicts without resorting to war.
People behave differently during a time of war.
War could break out soon. [=a war might start soon]
The two countries were at war (with each other).
The President decided against going to war. [=starting a war with another country]
The taking of American hostages was seen as an act of war by the United States.
2 : a situation in which people or groups compete with or fight against each other
3 [count] : an organized effort by a government or other large organization to stop or defeat something that is viewed as dangerous or bad
all's fair in love and war
this means war
— used to show you are offended by what someone has said or done and want to start a fight with that person
He insulted my girlfriend, and as far as I'm concerned, this means war.
(humorous) Make fun of my car, will you? This means war!