In every war Germany has fought with the UK, the Royal Navy has been able to cut off Germany from oceanic trade. WWII was no different. On September 3rd, two days after Germany invaded Poland, Great Britain and France used their navies to stop the passage of trade vessels to German ports.
The initial phase of the naval blockade ran from 1939 to 1940 and saw Germany cut off from direct trade with the Americas, Africa, and Asia. However, under the provisions of the Molotov-Ribbentrop Pact, Germany was able to purchase food and fuel from the USSR, and this lessened the effects of the blockade on Germany’s war-making capabilities. In June of 1940 Germany invaded and occupied France, thus gaining access to more ports and to a land border with Spain. Spain could import war materials and then sell them to the Germans, and could also sell domestically produced tungsten and aluminum and transport it overland. Germany also extracted natural recrudesces and food from France, which further helped the nation make up for domestic shortfalls. However, German attempts to directly combat the blockade generally failed as did attempts to carry out a blockade of Great Britain using submarines. In 1941 the situation worsened for Germany with the invasion of the Soviet Union. Germany could no longer import resources from the east and further failed to effectively exploit the areas of the USSR it occupied. Once the allies liberated France and the Balkans, Germany could no longer extract resources from those countries. Over the course of the war, Germany suffered from ever-worsening shortages of food, oil, steel, and numerous other materials. These shortages decreased the output of German industry and contributed to the collapse of the German Army in 1945.
WWII was fundamentally an economic war. Germany invaded nations in order to secure raw materials and land with the objective of achieving national self-sufficiency. However, Germany’s attempt to end reliance on other nations only led to shortages and failed in the end. In contrast, Great Britain made amazing use of its colonies and allies and was able to produce more weapons than Germany even with a smaller industrial base at the beginning of the war. In the end, reliance on international trade and specialization was not a weakness; rather, it allowed nations to make the best use of their own capabilities.