Its Shari'ah, Stupid #3

"If all men failed to perform the duty so that no able-bodied man went forth to battle, all I am afraid, would fall into error."
Jihad is an obligation; it must be done according to Allah's commands. What commands? Those cited in the Shari'ah.

• 9:5. Then when the Sacred Months (the Ist, 7th, 11th, and 12th months of the Islamic calendar) have passed, then kill the Mushrikun (see V.2:105) wherever you find them, and capture them and besiege them, and prepare for them each and every ambush. But if they repent and perform As-Salat (Iqamat-as-Salat), and give Zakat, then leave their way free. Verily, Allah is Oft-Forgiving, Most Merciful.  
• 9:29. Fight against those who (1) believe not in Allah, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allah and His Messenger (4) and those who acknowledge not the religion of truth (i.e. Islam) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued.
• 9:36. Verily, the number of months with Allah is twelve months (in a year), so was it ordained by Allah on the Day when He created the heavens and the earth; of them four are Sacred, (i.e. the 1st, the 7th, the 11th and the 12th months of the Islamic calendar). That is the right religion, so wrong not yourselves therein, and fight against the Mushrikun (polytheists, pagans, idolaters, disbelievers in the Oneness of Allah) collectively , as they fight against you collectively. But know that Allah is with those who are Al-Muttaqun (the pious - see V.2:2).
• 9:38. O you who believe! What is the matter with you, that when you are asked to march forth in the Cause of Allah (i.e. Jihad) you cling heavily to the earth? Are you pleased with the life of this world rather than the Hereafter? But little is the enjoyment of the life of this world as compared with the Hereafter. 
• 9:39. If you march not forth, He will punish you with a painful torment and will replace you by another people, and you cannot harm Him at all, and Allah is Able to do all things.
• 9:41.March forth, whether you are light (being healthy, young and wealthy) or heavy (being ill, old and poor), strive hard with your wealth and your lives in the Cause of Allah. This is better for you, if you but knew.
• 9:112. (The believers whose lives Allah has purchased are) those who repent to Allah (from polytheism and hypocrisy, etc.), who worship Him, who praise Him, who fast (or go out in Allah's Cause), who bow down (in prayer), who prostrate themselves (in prayer), who enjoin (people) for Al-Ma'ruf (i.e. Islamic Monotheism and all what Islam has ordained) and forbid (people) from Al-Munkar (i.e. disbelief, polytheism of all kinds and all that Islam has forbidden), and who observe the limits set by Allah (do all that Allah has ordained and abstain from all kinds of sins and evil deeds which Allah has forbidden). And give glad tidings to the believers. 

• 9:111. Verily, Allah has purchased of the believers their lives and their properties; for the price that theirs shall be the Paradise. They fight in Allah's Cause, so they kill (others) and are killed. It is a promise in truth which is binding on Him in the Taurat (Torah) and the Injeel (Gospel) and the Qur'an. And who is truer to his covenant than Allah? Then rejoice in the bargain which you have concluded. That is the supreme success .

What do believers do and why?

• Fight
• kill
• are killed
• in Allah's cause
• to make Islam dominant.

What will Allah do to them if they do not join the Jihad?

• Inflict painful punishment {Cast them into Hell.}
• replace them with someone else.