But as a military reformer, Yakir was dedicated and remarkable. He worked on the improvement of the Red Army until his demise. On 10 June 1937, only 2 days before his execution, he wrote an extensive letter to Nikolay Yezhov, head of the NKVD, about his observations and the important duties in the field of military. After his death, Stalin's great purge wiped out large number of the officers who had served under him. Many of Yakir's achievements, including his reforms and preparations for guerrilla activities in the event of an invasion of Ukraine, were dismantled. When Germany invaded the Soviet Union in June 1941, the Red Army was largely incapable of modern warfare and unprepared to face an enemy who used military art which Yakir and other Soviet innovators were greatly familiar with. The Soviets suffered terrible defeats and huge human and territorial losses before remastering modern operational approaches and tactics. Yakir's disciples who survived the purge used the experiences which they had gained under Yakir to make a vital contribution to Soviet victory over Germany. Among them were Chief of the General Staff of the Red Army Aleksei Antonov, Front commanders Andrei Yeremenko and Ivan Chernyakhovsky, and Army commander Alexander Gorbatov.