The first Gulf War had UN security council approval. The 2nd Gulf War was approved in a way that provided employment to legal pedants and more importantly, just enough political cover for the Chinese and Russians back home and with their low-rent allies to avoid their veto.
The New Zealand Greens cannot go on about multinational responses than oppose wars with UN security council approval.
Naturally, the Greens opposed NZ participation in the Afghanistan war despite its clear-cut UN authorisation. NATO and other mutual defence treaties were specifically triggered after 9/11.
The USA and others were and still are at war with Al-Qaeda; they can use force against that enemy and those who harbour them. The Taliban was warned. A Wiki has this nice quote by Stone (1921):
When the territorial sovereign is too weak or is unwilling to enforce respect for international law, a state which is wronged may find it necessary to invade the territory and to chastise the individuals who violate its rights and threaten its security.
The 9/11 terrorists were air pirates. NATO and allied military entered Afghanistan to subdue the home base of these brigands and those that harboured them:
- Naval and military deployments against pirate’s lairs date back thousands of years.
- The first war of the USA was with the Barbary Pirates in 1801 to 1805, with another war in 1815. These pirates waged war against the shipping of other nations, seized cargoes and ships, and sold captives into slavery.
- Punitive expeditions against bandits were commonplace too, such as chasing Pancho Villa and his gang of bandits back into Mexico in 1916.
- The U.S. military recently attacked a Somalian maritime pirate camp to rescue hostages. EU naval forces have also attacked these pirate lairs to destroy boats and supplies.