What is the ruling for those fighting for the sake of Allah?

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If they are traveling there, they can use the concession of the traveler to break their fast.

If they are fighting locally and they become exhausted while fasting, they can use the concession for those under extreme exhaustion; however, they still need to make up the days.

The opinion that says that they are not required to fast from the beginning, in order to preserve their energy and strength, is stronger and fits their need to break the fast better.

The evidence for this is the following hadith reported by Qaza:
I came to Abu Sa’id al-Khudri (ra) and he was surrounded (by people), and when they dispersed I said to him: “I am not going to ask you about what these people were asking. I ask you about fasting on a journey.” Upon this he said: “We travelled with the Prophet (pbuh) towards Mecca and we had been observing fast. We halted at a place. There the Prophet (pbuh) said: ‘You are nearing your enemy and breaking of fast would give you greater strength, and that was a concession (given to us).’ But some of us continued to observe the fast and some of us broke it. We then got down at another place and he (the Holy Prophet) said: ‘You are going to encounter the enemy in the morning and breaking of the fast would give you strength, so break the fast.’ As it was a point of stress, so we broke the fast. But subsequently we saw ourselves observing the fast with the Prophet (pbuh) on a journey.” [Sahih Muslim]

Some broke their fast once the Prophet (pbuh) gave them the concession the first time; others broke it later on after they were experiencing fatigue and were reminded again of the concession by the Prophet (pbuh); others broke it even later than that when many were becoming extremely exhausted. The Prophet (pbuh) himself broke his fast at that point and told the others that they must break theirs as well. A few decided that this did not apply to them because the Prophet (pbuh) only broke his fast to set an example for the others who were getting extremely exhausted. The Prophet (pbuh) described those few as being the “disobedient ones.”