KIEV HAS been the scene of a somewhat farcical drama this month centering on Mikheil Saakashvili, the former Georgian president-turned-Ukrainian dissident. Mr. Saakashvili was invited to Ukraine in 2015 by newly elected President Petro Poroshenko to assist in transforming the former Soviet republic into a Western-style democracy capable of someday joining the European Union. Now Mr. Poroshenko is doing his best to jail or deport his former ally — and repeatedly failing, thanks to demonstrations by Mr. Saakashvili's supporters and, most recently, a judge's ruling.

Mr. Saakashvili, who led a revolution in Georgia, now aspires to do the same in Ukraine. His attempts to do so were going nowhere until the government first stripped him of his Ukrainian citizenship while he was out of the country, then tried to arrest him on dubious charges of conspiring with a pro-Russian oligarch. The resulting street theater — Mr. Saakashvili was wrestled out of a police van by his supporters at one point — has served only to draw attention to the case he has been making: that Mr. Poroshenko's government, which promised to put an end to the endemic corruption that has haunted Ukraine since it became independent in 1991, has instead become a major obstacle to reform.

While police were pursuing Mr. Saakashvili, Mr. Poroshenko's supporters were conducting a lower- ­profile but more consequential campaign against reformist parliamentarians as well as the National Anti-Corruption Bureau, an agency set up with Western backing. An undercover investigation against a senior immigration official was disrupted, and the homes of a dozen anti-corruption officials and their relatives raided.

Strong protests from the European Union, the State Department and the International Monetary Fund slowed the offensive. But Mr. Poroshenko appears to be calculating that neither Western leaders nor Ukrainians — who overthrew the country's last, spectacularly corrupt president — will hold him accountable, because of Ukraine's ongoing standoff with Russian-backed separatists in its eastern provinces.

That low-level war simmers on: The United Nations reported Tuesday that 15 civilians had been killed in near-daily skirmishing between mid-August and mid-November. To the extent that it pays heed to Ukraine, the Trump administration is focused on the conflict: A U.S. envoy is trying to turn a Russian proposal for peacekeepers into a viable plan for pacifying the region. Secretary of State Rex Tillerson sees a security settlement in Ukraine as a prerequisite for normalizing relations with the regime of Vladi­mir Putin.

Such a deal would help Ukraine stabilize. But as long as it fails to clean up its institutions and firmly embrace the rule of law, it will remain deeply vulnerable to Russian subversion. As it is, Mr. Poroshenko's apparent defense of the old, corrupt order is increasing public sympathy for Russia and its proxies. Surely no one is enjoying the Saakashvili drama more than Mr. Putin, who with crocodile tears said Thursday that it "makes one so sad to see all this."

The United States and the European Union have considerable leverage over Mr. Poroshenko, who is still seeking defensive weapons from Washington and a pathway to E.U. membership. They should insist that he stop targeting anti-corruption investigators and create a special anti-corruption court. Meanwhile, Ukraine's leader should do himself a favor and stop pursuing Mr. Saakashvili. That campaign could only help the Georgian fulfill his misguided dream of becoming a revolutionary leader — or a martyr.