Word or phrase for premeditated act of self defense

Solution 1:

This is normally called a pre-emptive strike.

adjective
serving or intended to pre-empt or forestall something, especially to prevent attack by disabling the enemy:
a pre-emptive strike

Solution 2:

The phrase anticipatory self-defense has been used in international law since the 1837 Caroline Affair between the United States and Britian:

This incident has been used to establish the principle of "anticipatory self-defense" in international politics, which holds that it may be justified only in cases in which the "necessity of that self-defense is instant, overwhelming, and leaving no choice of means, and no moment for deliberation".

Solution 3:

The current term for this kind of anticipatory strike is proactive.