Whoever is compelled to break his fast for a valid reason, such as donating blood to save a life, and he or she does not have the strength to do so without food, the person is allowed to break his fast. Indeed, it is mandatory for him to break, because saving a life is mandatory. As they say: Mala, yatimul wajib illa bihi fahuwa wajid. (An action necessary to accomplish a mandatory act is mandatory.) Similarly, the person who needs to break his fast to gain strength to participate in defending Muslim life and property should break his fast.
In a hadith reported by Abi Sa'ed Al-Khudri (raa) he said: "We journeyed with the Messenger of Allah to Makkah while observing the fast of Ramadan. When we stopped at a place, he said, `You are getting near your enemy, and breaking fast will help you gain your strength.' This was a concession. Some of us continued to fast, while others broke their fast. Then, we stopped again, and the Messenger of Allah (saas) said, `You are meeting your enemy in the morning, and breaking fast is better for your stamina.' `Break!' That was an order. So, we broke." (Muslim/Ahmed) This hadith indicates how the desire to regain strength is an independent reason, besides a journey, to permit someone to break his fast.