Countries that remained neutral in WWI included Denmark, Holland, Sweden and Spain. Other countries that did not take part in WWI were Argentina, Chile, Afghanistan and Persia.
As most of the war took place in Europe, it was very difficult for countries not to become involved in the war. America tried to remain neutral during the war but Germany had a policy of attempting to sink any ships in war-zone waters and as a result, several American ships were sunk. The war began in 1914 and lasted for four years. The Allies and Germany signed an armistice on November 4, 1918.