Russia and other parts of the former USSR commemorate one of the darkest days of their history on Wednesday, the 70th anniversary of Hitler's invasion.

It has long been known that Stalin received warnings of an impending attack, prompting one of the great questions of military history: why were Soviet forces, despite their impressive numbers, so ill-prepared to withstand the Nazi blitzkrieg? 

Some accounts of the war have sought to play down the amount of intelligence the Kremlin had to go on, but this week a wealth of damning detail has emerged in the Russian media.

In an interview with Komsomolskaya Pravda newspaper, Russian military historian Arsen Martirosyan revealed that Soviet intelligence had named the exact, or almost exact, date of the invasion 47 times in the 10 days before Germany struck. 

It seems that Hitler himself all but let the cat out of the bag in May 1941, when he sent a letter to Stalin, who at the time was still a nominally friendly leader under the Molotov-Ribbentrop non-aggression pact....