Such ports as Rotterdam, Amsterdam, and Copenhagen are just as useful to Germany for purposes of commerce as are Hamburg and Bremen, and, in fact, a special commercial arrangement with Rotterdam has made that city practically a port of Germany since 1868.

These considerations show how ineffective would be a blockade of the German coast which did not also comprehend the coast of Holland and Denmark.
Germany could still conduct her commerce through these neighbouring countries.

And at this point the great difficulty arose.

A blockade is an act of war and can be applied only to a country upon which war has been declared.

Great Britain had declared war on Germany and could therefore legally close her ports; she had not declared war on Holland and Denmark, and therefore could not use the same measure against those friendly countries.