Final answer:
The cause of the Allies being unable to ship war supplies to Russia's ports was the blockade of Russian ports.
Step-by-step explanation:
The cause of the Allies being unable to ship war supplies to Russia's ports was the blockade of Russian ports. During World War I, naval blockades were established to prevent necessary war-related materials from entering countries. Great Britain, with its superior navy, instituted a blockade of German ports, making it difficult for Germany to import the goods it needed.