As bad deals go, it’s hard to do worse than the “Minsk II” cease-fire signed in February between Kiev and Moscow. First the Kremlin reneged on its promises to stop fighting by Russian forces and Kremlin-backed rebels in eastern Ukraine. Now Kiev’s attempts to implement its half of the deal are sparking a political crisis among the good guys in this fight.

That was clear after Monday’s grenade attack outside the Parliament building in Kiev, which killed three and injured dozens. The attack appears to be an isolated incident, but it highlights the extent to which Kiev is struggling to maintain law and order as nationalist discontent rises and the government fights a war that the Minsk agreement was supposed to have ended.

The attack, allegedly carried out by a member of a Ukrainian paramilitary group, happened at a protest against a constitutional amendment that would devolve more power to local governments. The idea isn’t controversial, and it was a condition of the agreement that five political parties reached last year to form the current governing coalition. Minsk II requires Kiev to legislate devolution before the end of the year.

The problem is that Ukraine President Petro Poroshenko is under pressure from his Western allies to abide by Minsk even as Russia’s President Vladimir Putin violates it. Mr. Poroshenko’s critics object to amending the constitution during a war and say the amendment as written risks giving too much autonomy to Donetsk and Luhansk, the eastern regions occupied by Russian-sponsored rebels.

Mr. Poroshenko’s coalition has started to fracture, with the smallest of the five parties in the government withdrawing Tuesday. The remaining parties still enjoy a majority, so a collapse isn’t imminent. But the episode illustrates the extent to which Mr. Poroshenko is paying a mounting political price for being the only leader honoring his end of the bargain.