question #3 Sahih Muslim 839a, Book 6, Hadith 370 : How does the concept of completing rak’ahs apply in prayers performed under challenging circumstances?

The Messenger of Allah (may peace he upon him) led on* of the two groups In one rak’ah of prayer in danger, while the other group faced the -enemy. Then they (the members of the first group) went back and replaced their companions who were facing the enemy. and then they (the members of the second group) came and the Messenger of Allah (ﷺ) led them in one rak’ah of prayer. Then the Messenger of Allah (ﷺ) pronounced salutation, and then they (the members of the Ant group) completed the rak’ah and they (the members of the second group) completed the rak’ah.