WaPo (“Russia warns U.S. to stop arming Ukraine“):

Russia this week sent a formal diplomatic note to the United States warning that U.S. and NATO shipments of the “most sensitive” weapons systems to Ukraine were “adding fuel” to the conflict there and could bring “unpredictable consequences.”

The diplomatic démarche, a copy of which was reviewed by The Washington Post, came as President Biden approved a dramatic expansion in the scope of weapons being provided to Ukraine, an $800 million package including 155 mm Howitzers — a serious upgrade in long-range artillery to match Russian systems — coastal defense drones and armored vehicles, as well as additional portable anti-air and antitank weapons and millions of rounds of ammunition.

The United States has also facilitated the shipment to Ukraine of long-range air defense systems, including Slovakia’s shipment of Russian-manufactured Soviet-era S-300 launchers on which Ukrainian forces have already been trained. In exchange, the administration announced last week, the United States is deploying a Patriot missile system to Slovakia and consulting with Slovakia on a long-term replacement.

Shipment of the weapons, the first wave of which U.S. officials said would arrive in Ukraine within days, follows an urgent appeal to Biden from Ukrainian President Volodymyr Zelensky, as Russian forces were said to be mobilizing for a major assault on eastern Ukraine’s Donbas region and along the coastal strip connecting it with Russian-occupied Crimea in the south. Russian troops have largely withdrawn from much of the northern part of the country, including around the capital, Kyiv, following humiliating defeats by the Ukrainian military and local resistance forces.

“What the Russians are telling us privately is precisely what we’ve been telling the world publicly — that the massive amount of assistance that we’ve been providing our Ukrainian partners is proving extraordinarily effective,” said a senior administration official, who spoke on the condition of anonymity about the sensitive diplomatic document.

We’ve been training and arming the Ukrainians since at least 2008, apparently to great effect, and have continuously ramped up our direct support since the most recent invasion. Arming belligerents in a conflict is an act of war and Russia is certainly within its rights to treat it accordingly. We are, for all intent and purposes, conducting proxy war.

Putin had judged, thus far correctly, that invasions of non-NATO countries (Georgia in 2008, Ukraine in 2014 and again in 2022) would fall short of a red line that would trigger US/NATO direct military response. The Biden administration and its NATO allies have judged, thus far correctly, that crippling sanctions and massive support short of actually getting kinetically involved in the war would stave off direct military confrontation with Moscow. But the problem with fighting in the so-called gray zone is that it’s quite possible to miscalculate where the red line is.

Given that Russia is getting its ass kicked in Ukraine, I’m reasonably confident that Putin isn’t going to expand the fight in such a way as to force NATO’s hand here. But it’s not out of the question.

Russia experts suggested Moscow, which has labeled weapons convoys coming into the country as legitimate military targets but has not thus far attacked them, may be preparing to do so.

“They have targeted supply depots in Ukraine itself, where some of these supplies have been stored,” said George Beebe, former director of Russia analysis at the CIA and Russia adviser to former vice president Dick Cheney. “The real question is do they go beyond attempting to target [the weapons] on Ukrainian territory, try to hit the supply convoys themselves and perhaps the NATO countries on the Ukrainian periphery” that serve as transfer points for the U.S. supplies.

If Russian forces stumble in the next phase of the war as they did in the first, “then I think the chances that Russia targets NATO supplies on NATO territory go up considerably,” Beebe said. “There has been an assumption on the part of a lot of us in the West that we could supply the Ukrainians really without limits and not bear significant risk of retaliation from Russia,” he said. “I think the Russians want to send a message here that that’s not true.”

The United States has certainly made those calculations in our wars. During the Vietnam War, we targeted sanctuaries in Laos and Cambodia. During the long war in Afghanistan, we made frequent incursions into Pakistan—at one point terming the theater AfPak. But, of course, those were comparatively minor players and the risk was low.

For Putin to attack NATO countries would trigger Article 5 consultations and, almost certainly, escalation. It would be madness. That doesn’t mean it’s impossible.