The 9/11 attacks seem to contradict the primacy of the state in international relations that was established by the Peace of Westphalia. How do...

One of the main theoretical perspectives in the study of international relations is called realism.  According to this theory, states (laypeople would call these countries) are the only really relevant actors in international relations.  We can say that the terrorist attacks of September 11, 2001 contradict this particular tenet of realism because they were carried out by actors who were not part of a state.

For a long time, realism was the dominant theory in international relations (IR).  Realism looks only at states as the major actors in IR.  This perspective argues that larger organizations, like the UN, and non-state actors, like NGOs, are largely irrelevant.  All states exist in an anarchic world in which they have to gain and hold enough power to survive. 

The terrorist attacks of 9/11 were a very important event.  They changed the way that the US engaged with the world.  On September 10, 2011, Afghanistan and Iraq were not major concerns of US foreign policy.  Two days later, Afghanistan certainly was and Iraq soon would be.  Even though the attacks had this much of an impact, they were not carried out by state actors.  No country’s military attacked the US that day.  Instead, the attacks were carried out by a terrorist group, Al-Qaeda.  In other words, the actions of a relatively few people unaligned with any state changed the course of world history and the way in which the US interacted with the rest of the world.  This contradicts the idea that states are the primary actors in IR.