The use of Russian-manned artillery inside Ukraine is being portrayed as a "significant escalation" in Vladimir Putin's effort to seize his neighbor's territory. That's putting it mildly. So far in this crisis the Russian strongman has practiced a form of ambiguous aggression—the insignia-less "little green men" in Crimea; the quasi-covert military aid to the separatists in eastern Ukraine—that provided the Kremlin with at least a fig leaf of deniability. What's happening now looks like an outright invasion.
The insertion of artillery, which was confirmed Friday by NATO officials, comes as a convoy of some 200 Russian trucks illegally entered the separatist-controlled territory in and around the cities of Donetsk and Luhansk. The trucks, ostensibly delivering humanitarian aid, were supposed to be escorted by the International Red Cross. Red Cross officials refused to join the convoy for fear of being caught in a crossfire, but the convoy entered anyway.
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Global View Columnist Bret Stephens on news that Russia artillery units are inside Ukraine, firing on Ukrainian forces. Photo: Associated Press

Exactly what Mr. Putin hopes to achieve remains to be seen. At a minimum, the convoy serves the Kremlin's domestic propaganda purposes by offering visual evidence that Mother Russia will come to the aid of fellow Russians stranded in the country's "near abroad" and under dire threat from allegedly nefarious forces.
U.S. Gen. Philip Breedlove, NATO's Supreme Commander, has noted that Russia has "previously sent 'humanitarian' and 'peacekeeping' efforts to Georgia, Moldova and Crimea, and we have seen how they proved to be deceptions that freeze conflicts rather than resolve them." The Kremlin formula is to insert the convoy, demand a ceasefire, then insist that Kiev honor the ceasefire, in turn allowing the rebel enclaves to become self-governing territories.
But the convoy also creates the possibility of an incident—accidental or premeditated—that can spark a wider war. Mr. Putin has a history of using such incidents to start wars against his enemies. That includes the mysterious apartment building explosions—blamed on Chechen terrorists but widely suspected of being the work of Russian intelligence services—that sparked the Second Chechen War in 1999 and first brought Mr. Putin to power. The 2008 invasion of Georgia was sparked by another ambiguous border incident.
As for the Ukraine crisis, there is little doubt the Kremlin is ready and perhaps eager for another incident. Russian Defense Minister Sergey Shoygu has "guaranteed" to the Obama Administration that the convoy would not be used to start an invasion. Yet his ministry has also stationed some 18,000 troops on the border with Ukraine, and now the deployment of Russian artillery shows how little that guarantee was worth.
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Russia's President Vladimir Putin in Yalta, Crimea, in August. Reuters

All the more so since Kiev has surprised much of the world, perhaps including itself, by prosecuting a successful military offensive that seemed to be on the cusp of cutting off the rebels from their supply routes to Russia. Among the reasons the Obama Administration has refused to supply Ukraine with arms is the fear that its military was incompetent, undisciplined and possibly disloyal. Having proven the Administration skeptics wrong, Ukraine's military deserves immediate U.S. support.
We noted last week ("Some Realism on Russia," Aug. 16) what some of that support might be: body armor, night-vision goggles, small UAVs, antitank weapons, shoulder-fired anti-aircraft missiles and radio jammers. This equipment can be rapidly loaded on C-17 cargo planes and flown directly to Kiev, much like the crucial aid that was delivered to Israel during the 1973 Yom Kippur War.
Merely the sight of those planes might give Mr. Putin reason to think twice about sending in the main body of his forces. It would also give Ukrainians—not to mention nervous NATO allies in the Baltics and Central Europe—confidence that Mr. Obama's assurances are more than talk. The President has boasted about the efficacy of his post-Crimea sanctions, but so far they've had little impact on the Russian economy and even less on the Kremlin's behavior, save perhaps to underscore how reluctant the West is to punish the Kremlin.
Eastern Ukraine is now the place where Western resolve is being acutely tested against the usual temptations of timidity and indifference. This is an old story, and Mr. Obama is fond of saying that this kind of aggression has no place "in the 21st century." But Russia's revanchism is a reminder that human nature remains the same no matter what century we're living in. Dictators do not do off-ramps. Their aggression doesn't stop until it is checked.
The White House on Friday called Mr. Putin's actions a "flagrant violation" of Ukraine's sovereignty. But the question now in Ukraine, as also regarding ISIS in Iraq, is not the sincerity of Mr. Obama's indignation. It's whether this President has the will to do anything to stop it.