[When Does Jihād Become Obligatory upon the Imams?]

Answer: Jihād by killing and fighting becomes obligatory upon the Imams when they possess the equipment and numbers required by the circumstances—on the condition that there is an expectation of achieving a preponderant benefit.
The evidence for what we have mentioned is the saying of Allah the Exalted: “Allah does not burden a soul beyond its capacity” [Al-Baqarah: 286], and: “So fear Allah as much as you are able” [At-Taghābun:16]. If the leaders of the Muslims do not possess the equipment and numbers necessary for jihād, the obligation falls from them. Hence the saying of the Commander of the Faithful Imām ʿAlī (Peace be Upon Him), as mentioned in the Sermon of al-Shaqshaqiyyah: “By the One who split the grain and created the soul, had it not been for the presence of the people and the establishment of proof by the existence of supporters, I would have cast its rope upon its shoulder, and would have given its last to drink from the cup of its first …”
As for stipulating the expectation of achieving a preponderant benefit, that is because of the known obligation upon the Imams to consider what benefits Islam and the Muslims and strengthens their position.
Thus, if the rulers have no hope of victory, nor expectation of it, but rather know or presume loss, then jihād should not be undertaken—indeed, it is not permissible—because it is not in the interest of the Muslims for their armies to perish. Allah the Exalted has said: “And do not throw yourselves with your own hands into destruction” [Al-Baqarah:195].
And the Commander of the Faithful Imām ʿAlī (Peace be Upon Him) said in al-Shaqshaqiyyah: “I looked, and behold, I had none with me except the members of my household, facing death with them …” etc.
If it is said: Al-Ḥusayn and Zayd and others (Peace be Upon Them) rose for jihād without possessing the necessary equipment and numbers, and without expecting victory.
We say: As for al-Ḥusayn and Zayd (Peace be Upon Them), they did not rise until they believed they had acquired the necessary equipment and numbers, and they expected victory and dominance. However, what they expected did not occur, for the equipment and numbers betrayed them at the decisive moment, in the time of hardship and at the point of apparent victory. Thus, they were compelled to defend themselves out of necessity.
The jihād of Muḥammad ibn ʿAbd Allāh and al-Fakhkhī (Peace be Upon Them) was a defensive jihād, and likewise those similar to them who were in a state of small numbers and weakness.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3