was
The word "war" is the past tense of "sein", which means "to be" in English. It is used to describe past events or situations. For example, "Ich war in Deutschland" means "I was in Germany". It can also be used to express past states or conditions, such as "Das Wetter war schön" which means "The weather was nice".
The war was very hard.
In this sentence, 'war' is used as a past tense of the verb 'sein' which means 'was.' The word 'Krieg' means 'war' and hence makes this a valid sentence with 'war' as requested.
Once upon a time there was a king.
This example is typically used in storytelling. 'Es war einmal' translates directly into 'it was once,' but a more fitting translation would be 'once upon a time.'
He arrived at work too late.
In this daily-life example, 'war' is used to show that he 'was' late for work. 'Zu spät' means 'too late,' and 'Arbeit' means 'work.'