In the year 2001, USA was the target of a large scale terrorist attack organized by non-state actor. After some time, the US started to target countries where the terrorists, at the moment of attacks, were residing. Following the lead of the US, other states also launched military operations in the territory of another state against non-state actors without the consent of that state or without the decision of the UN Security Council. Being the target of a terrorist attack organized by non-state actors, the response of the US and other nations following its lead to the attack resulted in an academic turmoil over the interpretation of self-defense. However, not only proper interpretation of self-defense, but also application of the states' responsibility had to be addressed. Therefore, this book addresses state responsibility in cases of self-defense against non-state actors. The state is not only responsible for hosting terrorist, but also for launching military operations.