For the three centuries before the outbreak of the Great War, the British relied on a strong navy, allied with a relatively weak army, to keep its continental enemies at bay. Indeed, Admiral Nelson is said to have spent almost two years (1803-1805), without setting a foot ashore, aboard HMS. Victory, whilst on 'open naval blockade' duty against the French fleet off Toulon. So, when 1 broke out in August 1914, almost the first action taken by the British Government was to order a blockade of Germany by the Royal Navy. However, in 1908 the British had initiated the Declaration of London to establish the rules of what interventions in international trade would be allowed in time of war. Signed by all the major maritime nations - but never ratified by Britain - the Declaration permitted the free export of goods, subject only to mild restrictions, and the freedom of neutral waters. None-the-less, aware that Germany's coastlines could only be accessed through the North Sea, the British mounted its long-range blockade - 'economic warfare' - with the aim of sealing off the whole North Sea, and thus closing all the German ports to international trade.
Germany, on the other hand, had invested heavily during the pre-war years in a vast expansion of its Navy to counteract any action of this kind. Accordingly, the scene was set for a serious confrontation at sea, sooner or later. In the event, this turned out to be the strategically indecisive Battle of Jutland on 31st May 1916.
Yet another element in the equation was the neutral countries, some of which were determined to preserve their right to freely trade with all the combattant nations. Particularly active was the United States. It delivered to the British government a whole series of protests about the potential effects this blockade would have on international trade with the Central Powers, i.e. Germany and Austro-Hungary. The inevitable protests of the Central powers, soon turned to reciprocity, with the introduction of the campaign of 'unrestricted submarine warfare'. This immediately took much of the immediate international pressure off the British, as the neutral nations saw their ships and crews lost by the action of the U-boats. Also, at least initially, the anticipated approbation of the neutral countries - particularly, the Netherlands, Denmark and Sweden - was largely deferred by two measures. Firstly, the British policy of largely sticking to the conditions of the Declaration where it most concerned the neutrals. And, secondly, the negotiating a convention of preferential guarantees to the affected neutrals to cover their loss of exports to the Central Powers.
Imposing the blockade on Germany
By the end of 1914, the British had succeeded in their overall aim of sealing off the German North Sea ports. But, this had very little apparent effect on the economies of the Central Powers and the execution of the war on the Western Front, or elsewhere. The British were compelled to consider what further measures they could put into effect. In March 1915 the Allies (Britain, France and Russia - the United States became an 'Associated Power' in April 1917) 'crossed the Rubicon' and formally decided to prohibit all international import and export trade with the Central Powers, including that of the neutrals. To ensure the efficacy of this measure, the Allies also imposed on the northern European neutrals a virtual monopoly on trade in the prohibited items.
As 1915 went on, the expected catastrophic effect of economic warfare on the Central Powers did not materialise. Germany increasingly used its powerful industrial base, and expertise, to produce synthetic products (e.g. high explosives, fertilisers and even coffee). Also, there was a steady flow of agricultural products across the borders of the Central Powers and their neighbours, including, latterly, Romania. But, virtually all re-exports by Germany did stop by the end of 1916.
The earliest indications of the effect of the blockade on the battlefields of the Western Front, were to be seen in early 1917. These deficiencies accelerated appreciably when the United States Navy joined the blockade in April of that year. Indeed, the United States now became the prime instigator of an even more stringent blockade and the control of imports to the neutrals. Also, Switzerland was brought more closely into the Allies' net, and a state of virtual trade embargo was activated across Europe. However, some neutrals, notably the Netherlands, Denmark, Norway and Sweden, refused to co-operate, and their adjacent borders made the achievement of compliance difficult.
The Allied Blockade Committee
These set backs resulted in the formation of the Allied Blockade Committee in April 1918 and, eventually, all the neutrals, except the Netherlands, agreed to a compromise settlement, which lasted until the end of the War.
By the end of the War, the North Sea Blockade was only one component, if the most important, of a worldwide campaign of economic warfare on the Central Powers. Ships of the German Merchant Marine had been seized or interned all over the Seven Seas, and a naval blockade was also firmly established throughout the Mediterranean.
After the War, claims were made that economic warfare, led by the North Sea naval blockade, was a primary factor in the collapse of the Central Powers, and the military victory of the Allies over the Axis forces. This was probably an exaggeration. Although the blockade indubitably played a leading part, the flow of imports and exports did not fall by much more than half of the 1913 levels of about 20% of the Gross National Product. Also, the ability of German industry to synthesize many important products, certainly ameliorated some of the effects that this reduction in imports and exports caused. To what extent the Allied Blockade weakened the resistance of both the military and civilian populations of the Central Powers to the 1918 Influenza Pandemic, remains a moot point.
The general consensus is that, in the end, it was the hard won war skills of the Allied armies, navies and air-forces, backed by their superior industrial productivity, that fought their Central Powers counterparts to a standstill, and led to their eventual collapse on the Western Front and elsewhere.