Easy. It’s Marshal Grigory Kulik.

I'm sure it wasn't all his fault. It was his blind loyalty to Stalin that made him so incompetent.

He wasn’t always directly ruthless, he actually convinced Stalin to spare 160,000 Polish soldiers at Katyn, but the results of his other actions are to be noted.

He refused development of submachine guns, commenting that the new German MP-40 was a “Pure police weapon”, and that submachine guns “wasted ammunition”. He refused to allow his own troops to be equipped with PPD-40s (early Soviet SMG). This was a big problem in the Winter War, a war in which he was commanding armies in, since the Finns had plenty of submachine guns and most engagements were at close range (One of the many reasons Finland did well against the Soviets). Not allowing your troops to have anything other than bolt action rifles in close quarters combat is a recipe for total disaster.

He refused the development of the T-34 medium tank and the KV-1 heavy tank, and even opposed Marshal Tukhachevsky’s recommendations for armored warfare because they were similar to the German doctrine. Not only that, he was against tanks and armored vehicles altogether, stating that they were “inferior to horses and will never replace them”.

The T-34s and KV-1s at the time were armed with 76.2mm L-11 guns, which had very poor penetration, and had trouble taking down Panzer IIIs. A new gun, the 76mm F-34, proved to be much better in all aspects, but Kulik refused to endorse it. Stalin eventually endorsed it after receiving numerous complaints from tank crewmen on the front that the L-11 was a terrible gun.

Kulik also purposely restricted production of 76mm ammunition, so very few KV-1s and T-34s in the early war period had an adequate amount of ammo. Some tanks had to rely solely on their machine guns, as they had little to no ammo for their main guns. Even fewer tanks had armor piercing rounds, and only had a handful of high explosive rounds, which were not designed to face tanks. Many tanks were abandoned by their crews as they ran out of ammo.

He refused the development of the BM-13 rocket artillery system. It was eventually endorsed by higher ranking officials, and proved to be one of the most effective and deadly weapons invented by the Soviet Union. It was also a large advancement in rocket artillery technology.

He was against the use of landmines in defensive combat, stating that they were a “weapon of the weak”. Thanks to him, the Wehrmacht was able to move anywhere they wanted, and thousands of troops were encircled as a result.

His actions resulted in severe hindrance of the war effort, and this earned him a bad reputation. His colleagues described him as a “murderous buffoon”, and his subordinates were either decorated with mostly unearned medals (if he liked them), or sent to prisons and gulags on false charges (if he didn’t like them).

Had he not slowed the war effort, the war would’ve not lasted as long, and fewer casualties would’ve been suffered.

Eventually he was executed in 1950 on charges of treason.