Although he was a close ally of President Petro Poroshenko's new government in Kiev, neither had the financial nor military clout to achieve that aim. So Kolomoisky decided to build his own private army of volunteers, equipped with heavy weaponry. He paid for all of this from out of his own pocket.
The recruits came from Ukraine and Europe. There are even a couple of Americans. Estimates suggest Kolomoisky could call on over 20,000 troops and reserves. His Dnipro Battalion, also known as Dnipro-1, includes around 2,000 heavily armed fighters. The unit is reported to have cost the banking billionaire $10 million to set up. They helped play a key role in halting the advance of the Moscow-backed rebels from their strongholds in the neighbouring Donetsk and Luhansk.
However, there are doubts about where the troops' ultimate loyalties lie - to the government in Ukraine or to their regional paymaster. Last week, armed men in masks stormed the headquarters of state-owned oil company UkrTransNafta in the Ukrainian capital Kiev, following the sacking of its director Oleksander Lazorko, a key ally of Kolomoisky.
On Tuesday, Poroshenko fired Kolomoisky and now this private could become a major problem for the Ukrainian authorities.