There used to be a difference between nation (the people) and the state (the land). Since the rise of the modern nation-state that difference has all but disappeared, it seems.
Note that the traditional state (ref. French état) was also sometimes used to indicate the person ruling it (Louis XIV famously announced l'état, c'est moi, the state is me). It was not uncommon to refer to the kings of, say, England or France as simply England or France. We still do the same when we use someone's title (which basically means they rule a piece of land) as their name: Wellington beat Napoleon at Waterloo. Of course we do not mean that the village of Wellington in Somerset went to Waterloo, but rather Arthur Wellesley, 1st Duke of Wellington.
With that in mind, it is not all too strange to see people referring to nation-states when they probably mean the executive government of said nation state. It would probably make for tiring reading if you would want to discuss trade agreements between the negotiators acting on behalf of the government of the United States of America and the envoys negotiation on behalf of the president of the Russian Federation instead of an agreement between the USA and Russia.