Joseph Stalin was never openly or officially declared insane by any psychiatrists during his reign in World War II. However, according to Vladimir Lenin in The Great Terror: A Reassessment by Robert Conquest, he had strong doubts about Stalin and his mental stability. About a year prior to his death, he wrote a covert “political testament” in which he disclosed his worry regarding Stalin:

Stalin is too rude, and this defect, though quite tolerable in our midst and in dealings among us Communists, becomes intolerable in a General Secretary. That is why I suggest that the comrades think about a way to remove Stalin from that post and appoint in his place another man who in all respects differs from Comrade Stalin in his superiority, that is, more tolerant, more loyal, more courteous and more considerate of the comrades, less capricious.

If one were to adhere to the psychoanalytic contention that a person’s childhood strongly influences his or her personality, it is plausible to argue that Stalin’s violent tendencies developed as a result of his father’s behavior.

According to Psychotherapy and the Paranoid Process Hardcover by W. W. Meissner:

Strained relationships between fathers and sons, husbands and wives, and consequently differing goals for children set up conflict within families. Paranoia is said to oftentimes enter within a maladaptive relationship with the father.

There are numerous theories that may be used to explain Stalin’s personality, and although there may be credibility to them, it is important to not overestimate the influence of childhood environment and attribute this singular circumstance as the cause of possible psychopathology. The combination of paranoid personality disorder, alcohol abuse, intelligence, and a cruel nature certainly created the foundation for Stalin’s infamous mass killings.

Stalin stands out as one of the figures in history most coolly and casually willing to use violence. He could plausibly be accused of narcissism, paranoia and psychopathy, but really he was so careful to conceal his true persona behind that unwavering Uncle Joe grin he always wore that definite conclusions aren't possible. One thing he was not was insane. Even his maddest policies had rationales and precedents.

The great terror carried echoes of the Oprichnina of Ivan the Terrible both in form and content. The communist bureaucracy and the Red Army officer corps were the main targets which caused many of the people to view it as a benevolent attack on their oppressors. The impression was entirely false, but that didn't alter its political effects.

The near-genocidal campaign to extract grain from Ukrainian peasants was likewise carried out in pursuit of a simple and rational objective: money. The grain was exported to generate the hard currency needed to pay for imports of industrial equipment etc.

I've never heard he was incapacitated 'for weeks' after the German invasion. On the contrary he was all too active in interfering in military operations, a sphere in which he possessed indifferent skills. As fighter, though, and ultimately as a war leader, he was experienced, resilient and cool-headed. Soviet management of the war economy was outstanding and the USSR was capable of long-term planning of the sort Germany never got around to.