Blessed Eyes

The Messenger of Allah (sal Allahu alaihi wasallam) said: “Two eyes will not be touched by Hellfire: An eye that cries from the fear of Allah, and an eye that spent the night in a guard post in the path of Allah (Jihad).” [Tirmidhi]

This hadith shows the significance of being on the alert for the protection of Muslims.

When there is an Islamic state in which Allah's laws are supreme, not subordinate to man-made laws, then Jihad is Fard Kifayah (an obligation on the community and not on every single Muslim). This is because if Muslims give up Jihad altogether, the aspirations of the Kuffar to take over Muslim lands are emboldened. This is what has happened since the destruction of the Khilafah. Moreover, the justice of Islam is for all humankind. Others may not be left to suffer under oppressive man-made systems while guidance from Allah (subhana wa ta'ala) is present.
 
The four Fuqaha (jurists), the Ulema (scholars), the Mufassireen (scholars of tafseer), and the Muhaditheen (scholars of hadith), all agree, that if Muslim land is occupied by as much as a handspan, Jihad becomes Fard Ayn (obligatory on each and every Muslim in whatever capacity they can manage).

Furthermore, the Fuqahaa have stated that if a part of a Muslim land is unable to expel the enemy, either due to lack of resources or laziness or negligence, on the part of the Muslims living there, then the obligation spreads in a circular fashion to those surrounding them [Ibn Abidin, Ibn Nujaym, Ibn al-Humam, al-Kakaani]. This obligation will continue to spread until the obligation of repelling the enemy is fulfilled.

Another condition under which Jihad becomes Fard Ayn is if there is a single Muslim prisoner in the hands of the Kuffar.