Ukrainian President Petro Poroshenko on Feb. 13. (Michael Dalder/Reuters)

THE BEST way to help Ukraine fend off Russia’s aggression is to build a strong and prosperous nation. That has been the core of Western thinking in the nearly two years since Russia seized Crimea and staged a separatist uprising in the Donbass. Now, a fresh warning to Ukraine’s leaders has appeared, this time from within, challenging them to heal the country’s chronic corruption and oligarchic rot. They must listen or risk losing all.

The warning came from the economic development and trade minister, Aivaras Abromavicius, 40, a technocrat and dedicated reformer brought in 14 months ago to help make a decisive break with the past and point Ukraine toward a long-overdue economic overhaul under President Petro Poroshenko. “We came here to do big things like deregulation, state-owned enterprise reform, and public procurement reform,” Mr. Abromavicius said this month, adding that the nation’s leaders had promised him “absolute political support and commitment to reforms.”

It was always going to be uphill. Ukraine has remained mired in a system in which billionaires gobble up state assets and siphon off the revenue streams. This corrupt form of oligarchic capitalism grew rapidly in the years after the Soviet Union imploded — it flourished in Russia, too — but Ukraine’s variant has been particularly deep-rooted. Finally, on Feb. 3, Mr. Abromavicius announced he was resigning because Ukrainian plutocrats were grinding down his efforts at reform.

He declared that businessmen close to Mr. Poroshenko were seeking to infiltrate the oil and defense industries and attempting to put their own people in government and corporate posts. “It has become clear that any kind of systemic reform is decisively blocked,” Mr. Abromavicius said. “I refuse to be part of this system. Neither me nor my team are prepared to serve as a cover-up for the schemes, old or new, that have been set up in the private interest of particular political or business players.” He added, “Evil forces still want to wind things back. Let us get rid of all those who shamelessly siphon billions off the Ukrainian economy.”

Specifically, he identified Ihor Kononenko, a businessman who is a deputy faction leader in Mr. Poroshenko’s bloc in parliament, as an obstacle to reform. As economist Anders Aslund of the Atlantic Council pointed out, Mr. Kononenko is Mr. Poroshenko’s right-hand man and often called the party’s “gray cardinal” by journalists; the crisis now goes all the way to the top.

The outburst was a needed shock to the status quo in Ukraine, and the question now is whether Mr. Poroshenko will do anything about it. Ambassadors from the United States, Canada, Britain, Germany, France, Italy, Lithuania, Sweden and Switzerland demanded in a statement that Ukraine put the vested interests that have dominated it for so long “squarely in the past.” The International Monetary Fund has indicated that Ukraine’s fiscal lifeline could be cut off in the absence of clear government action. Ukraine’s leadership must not ignore the warnings. Oligarchic capitalism was a messy first phase after the collapse of communism, but to survive, Ukraine must bid it farewell.