Should the mujaahideen who are fighting for the sake of Allaah break their fast?

If those who are fighting the kuffaar have travelled the distance which allows them to shorten their prayers, then it is permissible for them not to fast, but they have to make it up after Ramadaan. 

If they are not travelling because the kuffaar have attacked them in their own land, then whoever among them is able to fast whilst fighting is obliged to fast, and whoever is not able to combine fasting with the duty of jihad which has become an individual obligation (fard ‘ayn) for him is permitted not to fast, but he must make it up later on, fasting the number of days that he did not fast in Ramadaan. End quote. 

Standing Committee for Academic Research and Issuing Fatwas