Firstly: Tiring Professions  See the footnotes for: ability to fast, in Section I.
Secondly: Intense Hunger and Thirst
Whoever is struck with intense hunger or thirst such that he fears he may die, then it is wajib to break his fast, and he must make it up. This has been explicitly mentioned by the Malikis and Shafi`is.
Thirdly: The Ruling of the Forced to Break their Fast
If a person is forced to break their fast, so he does, then there is no sin over him and his fast is valid. This is regardless whether breaking the fast was or wasn't his own action, like having water ejected down one's throat. This is a position among Shafi`is and Hanbalis.
Fourthly: Fighting for the Sake of Allah
It is permissible for the one fighting for the sake of Allah to break his fast. This is the position of the Hanafis, Malikis, and a narration from Ahmad. It is also the verdict Ibn Taymiyyah gave the Muslim soldiers when they met their enemy at Damascus, and Ibn al-Qayyim supported him.