AFTER AN inconclusive summit with Ukrainian leader Petro Poroshenko this week in Minsk, Belarus, Russian President Vladimir Putin declared that Moscow would do “everything” it could to encourage a peace plan for eastern Ukraine and “create an atmosphere of trust.” But within hours of uttering those words, a new front in the conflict opened at the port of Novoazovsk, where Russia appeared to be backing a stealthy but concerted invasion with tanks and artillery, a counteroffensive to help Russia’s besieged separatist fighters near Donetsk.

This is “trust”?

Mr. Putin’s blithe denials that Russia is supporting separatists in Ukraine are already stale. When he says “this is not our business,” as he did in Minsk, he is lying. But the bald untruths and military thrusts into eastern Ukraine provide a revealing glimpse of Russia’s new approach to throwing its weight around. Mr. Putin is engaged in subversion and feint, perhaps not surprising for a onetime lieutenant colonel in the Soviet KGB.

Some have called the new approach “hybrid war,” a conflict waged by commandos without insignia, armored columns slipping across the international border at night, volleys of misleading propaganda, floods of disinformation and sneaky invasions like the one into Crimea. In this hybrid war, a civilian airliner was shot down by surface-to-air missiles, but the triggerman or supplier of the missile was never identified; artillery shells are fired but no one can say from where; Russian military material and equipment appears suddenly in the villages and fields of eastern Ukraine. While people are being killed, as in any war, and while Ukraine has mustered its forces admirably to push back, this hybrid war features an aggressor whose moves are shrouded in ­deception.

Why has Mr. Putin done this? Mark Galeotti of New York University suggests that, in a world of alliances and powers that have greater military, political and economic clout than Russia, Mr. Putin needed an asymmetrical response. To keep Ukraine from signing up with the European Union, he used tactics that focused “on the enemy’s weaknesses and avoid direct and overt confrontations,” as Mr. Galeotti put it.

502 Bad Gateway

A new kind of war requires a new kind of response. The hybrid aggressor must be exposed on all fronts, not just in combat on the ground. The Obama administration has published satellite photos and fact sheets and joined with European allies to impose sanctions. These are useful but insufficient steps. Mr. Putin’s intentions are unknowable, but he seems committed to preventing Ukraine from charting its own course and he appears willing to play a long game. Unfortunately, his shadowy tactics have led some European leaders to relax and resist further sanctions on Moscow. This is shortsighted. Russia and its proxies are shooting their way into Ukraine using a zigzag route intended to confound and confuse. Only clear eyes and determination will stop them.