phrase meaning - Is the following usage of past progressive correct?

What is the difference between the past continuous and the simple past usage of these sentences?

1. They were fighting Nazis for three days.

And

2. They fought Nazis for three days.

Can we use them both interchangeably? Or would past continuous in #1 would be right?

Answer

Even though fight is understood to refer to a series of interactions which take time to run their course (the sense of the ongoingness is implicit on the semantic level), that does not prevent us from choosing a verb form which emphasizes the ongoing aspect grammatically.

The progressive marks the verb as action-in-progress.

The choice of verb form affects how we understand temporal phrases added to the sentence.

The dog salivated when the bell rang.

The dog was salivating when the bell rang.

In the first, the bell rings and then the dog salivates.

In the second, the dog was already salivating when the bell rang.