In modern terminology, and for foreign policy, political science and international law, crimes against humanity are any atrocious act committed on a large scale. They can be prosecuted in most any Federal Court System, depending on where they occurred and which population was part of the criminal activity. The implication for international law is that crimes against humanity are subject to universal jurisdiction, which means that States can exercise their own jurisdiction regardless of where the crime was committed, and that all States also have the obligation and duty to assist each other in the defense of these sorts of activities. It is also important to note that no human, regardless of affiliation, is immune from prosecution, even heads of state, and on person can plead a defense as obeying orders.

Part 3 -- Is it legally justified to invade/occupy another country in the name of arresting/hitting terrorists? International cooperation regarding the arresting or bringing to justice those suspected of being involved in terrorist activities began in the 1960s and revolved around airline hijackings. Several notable events occurred to solidify international laws: the 1985 seizure of the cruise liner the Achille Lauro, the hijacking of TWA flight 847, the 1972 Munich Olympics issue, at sea piracy, and of course, the events of 9/11.

There are really two issues surrounding this question: 1) international cooperation in the aiding of capture and extradition of suspected terrorists, and 2) the legality of entering a sovereign nation who refuses to cooperate internationally. First, countries that belong to the United Nations...

In this sense, it is legal to share in law enforcement efforts to capture suspected terrorists as long as the host country allows other military, para-military or law enforcement agencies to operate on their soil.
A combination of the United Nations, International law signatories, and the Hague Conventions are designed to adjudicate crimes against humanity and bring criminals to justice in the international scope. The United Nations Universal Declaration of Human Rights (e.g. 1948) does give a limited general authority to enter a sovereign nation if the United Nations legally decrees that nation is in violation of international law. For country a to simply send a force to country B. To extradite suspected terrorists without first engaging in International Law would not be legal without appropriate resolutions issued by a recognized international authority and would likely be considered an act of war. It is really the circumstances that justify what can happen -- with variables of breadth of the attack (how many countries involved), seriousness, potential for success, and the view of the international community.

Sources:

Dyson, W. (2012). Terrorism: An Investigator's Handbook. Waltham, MA: Anderson

Publishing.

McCormack, W. (2007). Understanding the Law of Terrorism. New York: Lexis Nexis.

Title of agency, not plagiarized