[4] He later began leading a division of the Ukrainian Defense Ministry tasked with improving the quality of the country's military equipment.

[5] In 2019, Ukraine's State Investigation Bureau began a probe into allegations that Marchenko engaged in corruption by providing the Ukrainian military with poor-quality equipment at inflated prices, including "malfunctioning and overpriced bulletproof vests".

Marchenko worked alongside regional governor Vitalii Kim during the city's defense during the battle of Mykolaiv.

Early into the battle, Marchenko predicted that his troops would be trapped; the city had no infrastructure to stop an invasion, such as trenches or checkpoints.

[5] Starting on July 27, "[serving] as a senior official for cooperation between the army, partisan resistance, and the regional military administration" in Mykolaiv and Kherson Oblasts.

[17] Meanwhile, Marchenko claimed that taking Kherson would mean an end to the "active phase of the war", and that Ukraine planned to liberate Crimea, Luhansk, and Donetsk (which had all been annexed by Russia previously).

[18] Ukraine destroyed local infrastructure important to the Russians, such as command centers, ammunition depots, and bridges on the Dnipro River which resupplied units in Kherson.

[18][19] On August 10, Ukraine damaged the bridge at Nova Kakhovka, which was the "last transport artery" for the nearby Russians.

[20][18] On August 12, CNN wrote that Zelenskyy's recent announcement of investigations into a "high-ranking military officer" may have referred to Marchenko.