When is "offensive Jihad" Islamically approved?

Translating Jihad notices a recent fatwa on IslamOnline.net, one of the most influential Muslim websites worldwide:

Fighting Against Non-Muslims -- Legitimacy and Rulings

islamonline.net, 8 Jan 2011

Q: When is it permissible to fight against non-Muslims, and when is it not permissible? Please elaborate.

Mufti: Dr. 'Imad Mustafa, Professor of Fiqh and Its Origins, at the Universities of al-Azhar and Umm al-Qary

In the name of God, praise be to God, and prayers and peace be upon the prophet of God, etc.:

Fighting against non-Muslims is what is known in Islamic jurisprudence as Jihad in the path of God. Jihad is a prescribed duty in cases of aggression from the infidels against Muslims, for we must resist them, make jihad against them, and defend against them. This is according to the text of the Qur'an, for Almighty God has said: "Fight in the way of Allah those who fight you but do not transgress. Indeed. Allah does not like transgressors" (Qur'an 2:190). This type of jihad is known as defensive jihad, and it is a duty agreed to by all Islamic scholars and all who are wise, and is endorsed in our day by recognized international charters. However, the occupier and his associates have come to label this "terrorism."

Then there is another type of fighting against the non-Muslims known as offensive jihad. Islamic scholars have differed on the issue of offensive jihad, which is to pursue the infidels into their own land without any aggression or assistance to him. Some scholars have gone as far to say that this jihad is illegitimate, while others have gone as far as to say that it is legitimate and even required.

However there can be no disagreement that offensive jihad is not totally prohibited, for two schools [of Islamic jurisprudence] have ruled that offensive jihad is permissible in order to secure Islam's border, to extend God's religion to people in cases where the governments do not allow it, such as the Pharaoh did with the children of Israel, and to remove every religion but Islam from the Arabian peninsula, and to save the captive and weak. [...]

Ny the way, Translating Jihad is a very worthwhile site to follow. Here is another good post from today.