When I talk about the King in chess, I don’t necessary talk about a person, but rather an abstract decisive issue that will make or brake a specific nations ability to survive. In German its named a Schwerpunkt, this strategically important point that the King in chess is supposed to represent. Lets call it a weakness, and if you successfully target this weakness, then that nation will die. In some cases this will be a dictator like Hitler or Stalin, or a king or other authorian ruler. Like when Alexander the Great died, the day after all of his vast empire vanished. In a democracy things are a bit different, like when Roosevelt died in 1945, USA was not knocked out of the war, they just replaced him with Truman. Now who could replace Hitler or Stalin ? I guess that if Churchill died or London got captured, then the Brits would keep on fighting, maybe from a government in exile in Canada and with some other guy as a new leader. In a Dictatorship the Leader is the weak spot, but in a Democracy the opinion is the weak spot. As in the Vietnam war, USA was military supreme in the battlefield, but still lost the war because the commies was able to target the US domestic opinion and create resistance against the war. Other weak spots can be industry, production, lack of oil, food or supply, convoy routes over the sea, international trade and 100 other things. Even the army or navy can be a weak point, if out of supply or with poor leaders.
So back to my statement. I stress that in this case, Stalin was Russias weak point that Hitler had to hit if he wanted to win the war. As others have already said, to conquer Leningrad, Stalingrad, Murmansk or Siberia would not matter, that would be like killing a lot of cheap prawns in chess. A lot of fun but don’t make a victory. But Hitler failed to recognize this. Hitlers aim, or national objective if you want, that he published with his book Mein Kampf in 1920, and won the 1933 election on, was to colonize the Eastern Europe to the Ural mountains, and simoultanesly ethnical cleanse about 30 million of the local people living there, making living space for the Ubermensch farmers. And this is basically why Hitler lost the Eastern Front, and in the end the war. Now, if Hitler had been cunning, skilled and smart, he would have gone directly for Moscow, with full strength and all resources, captured Moscow, jailed Stalin and demobilized the Red Army. Now Germany would have won the war, and with less then a half million germans as casualties, no cities in ruins, dominated all of Europe, and then Hitler could have started to kill Jews and Slaves and probably succeeded, much in the same way that Stalin succeeded in killing 30 million of his own population during the Great Terror in the 1930 ies. Its a lot easier to kill people when they are not armed and shooting back. Luckily to my family and country, Hitler was narrow minded and stubborn.
The Napoleon campaign 1812 was another kind of war, since Napoleons national objective was to win honor in the battlefield by chasing and beating the Russian Army. If Napoleon had wanted to conquer and govern Russia he could just have invaded the capitol, St. Petersburg, that was almost adjacent to his staging point, which would give him short supply lines, and the city was light defended too since the Tsar had his army around Moscow.
King Karl XII campaign in 1700 was different too, since his objective was to capture Tsar Peter I and steal his money, so he followed him to an ambush outside Poltava.