This is another article in the section Jihad from the classical manual on Sharia called Reliance of the Traveller. The articles are posted one by one in the order they are given in the book.
WHO IS OBLIGED TO FIGHT IN JIHAD
O9.4 Those сalled upon (0: to perform jihad when it is a communal obligation) are every able-bodied man who has reached puberty and is sane.
O9.5 The following may not fight in jihad:
(1) Someone in debt, unless his creditor gives him leave:
(2) or someone with at least one Muslim parent, until they give their permission;
unless the Muslims are surrounded by the enemy, in which case it is permissible for them to fight without permission.
O9.6 It is offensive to conduсt a military expedition against hostile non-Muslims without the Сaliph’s permission (A: though if there is no caliph (def: 025), no permission is required).
O9.7 Muslims may not seek help from non-Muslim allies unless the Muslims are considerably outnumbered and the allies are of goodwill towards the Muslims.