Abdullah Yusuf Azzam issued his fatwah Defense of the Muslim Lands, the First Obligation after Faith (1979) after the Soviet invasion of Afghanistan. Fatwahn supported by Saudi Arabia's Grand Mufti, Abd al-Aziz Bin Bazz. Azzam legitimize both defensive and offensive jihad.
http://www.religioscope.com/info/doc ... _chap1.htm
Notice the minimum requirement:the sending of an army at least once a year to terrorise the enemies of Allah. This is is agreement with Al-Shafi'i and other authorities. Notice also that offensive Jihad is fard Kifaya.