The international political system is anarchic,  

as there is no supranational  authority to enforce rules;

The often-used analogy of there being 'no one to  

call' in an international emergency  might help to underline this point.

Within our own countries, we typically have  police forces, militaries, courts, and so on.  

There is an expectation that these institutions  will "do something" in the event of an emergency.

However, there is no clear expectation of  anyone or anything 'doing something ' on a  

global scale because there is no  defined hierarchy. As a result,  

nations can only rely on themselves in the end.