ウクライナ人民共和国軍
In December 1917 he switched to Ukrainian service where his first assignment was the 2nd Serdyuk Division which was only starting to form and at that time consisted out a small officer corps. One of the Hrekov's first remarkable subordinates on the side of the Ukrainian People's Army was a colonel Petro Bolbochan. At one time the Russian prime-minister Vladimir Lenin was offering 50,000 rubles in silver for the head of colonel Bolbochan. During the same time Hrekov was appointed the chief of the Kiev Military District and was partially involved in the formation of the Sich Riflemen Halych-Bukovyna Kurin (Battalion) led by Yevhen Konovalets. The unit became the main supporter of the Central Rada and Mykhailo Hrushevsky. After the fall of Kiev early in 1918 Hrekov retired from the Ukrainian People's Army. From December 1918 until January 1919 he was assigned to undertake negotiations with French forces occupying Odessa on behalf of the Directorate of Ukraine. After a conflict with Petliura and leftist Ukrainian politicians, he was forced to resign his post and on 16 May 1919 assumed command of the Ukrainian Galician Army offered to him by Yevhen Petrushevych.