Would or would have fired

[tufguy]

They have gone there with guns, if necessary they will fire on enemies.

In past tense

"They left with guns, if necessary They would fire on enemies" or do I need to use " would have fired" here?

 

[MikeNewYork]

"Would have".

 

[tufguy]

But I am confused between these two could you please explain me how to know whether we have to use "would or would have done" when talking about past?

 

[MikeNewYork]

Piscean makes a good point, but I personally would stick with "would have fired" to keep the tenses straight.