http://www.familysecuritymatters.org/publications/id.11269/pub_detail.asp
Islam Commands Individual Jihad Clare M. Lopez
According to Islamic law, Jihad is warfare to spread the religion and is obligatory for all Muslims everywhere until such time as “there is no more Fitna [resistance to Islam] and the religion will all be for Allah alone.” (Q 8:39) Jihad against the infidel (kuffar) is of two types: offensive and defensive. The offensive kind, called “Fard Kifaya,” is conducted under the leadership of the Caliph and is for the purpose of defending the borders of Islam and sending out the armies of Islam at least once a year to terrorize the enemies of Allah. Offensive Jihad is a collective duty and when enough Muslim fighters respond to the call of the Caliph, the remainder of the Muslim population is relieved of this duty.
Defensive jihad is a compulsory duty upon all Muslims primarily when the kuffar enter Muslim lands. It is considered so critical to repulse the invader that Ibn Taymia even asserted it took precedence over making the hajj. This defensive jihad duty is called “Fard ‘Ayn” and means that there is a personal obligation for all Muslims in the land which has been attacked to join the fight. The scholars agree that children may march forth without permission of their parents, wives without permission of husbands, and slaves without permission of their masters. If the Muslims of the land attacked by the kuffar cannot expel the enemy for whatever reason, then the Fard ‘Ayn obligation expands outward to include other Muslims nearby and then at increasing distances from the center of the invasion. Eventually, Fard ‘Ayn can become obligatory across the entire world until the kuffar is expelled from Muslim land.