Certainly, the modem State is limited by national frontiers. We are members of the State within whose frontiers we are born and we can change our State allegiance or nationality only with the specific permission of the State to which we wish to belong. Still, the State is not synonymous with the nation: both are distinct in meaning and connotation.
The Scots and the Welsh would claim to be members of a nation, but they are members not of a Scottish or Welsh State, but of the British State. In the course of history, States have annexed nations. Alsace and Lorraine were, from 1870 to 1914, part of the German State. From 1918 to 1940, they formed part of France and from 1940 to 1945; they again formed part of Germany.
Today, they are once more part of France. If you ask an Alsatian whether he is French or German, he will reply that he is Alsatian and he may add that he is also French as he is a citizen of the State of France.
A mere organisation of the people under one government does not make them a nation. Austria-Hungaiy, before World War I, was a State, but not a nation. Inhabited by heterogeneous people, there was nothing other than political bonds which could knit them together in ties of oneness.
Then, sovereignty is the most essential feature of the State whereas the people may continue to be a nation even if they do not retain their sovereign character, either as a result of conquest or annexation of their country.
Germany and Japan no longer remained States, after World War II in 1945, though the Germans and the Japanese were still nations. Poland and Finland, before World War I, were nations, though not States. The term nation signifies the consciousness of unity reinforced by psychological and spiritual feelings. Nationhood is, therefore, subjective while Statehood is objective and political.