WASHINGTON — America’s top military official in Europe called the use of a chemical weapon to assassinate a former spy in the United Kingdom “amazing,” and said it “underscores” the lengths Russia will go to achieve its objectives.

Gen. Curtis Scaparrotti, head of U.S. European Command and the Supreme Allied Commander of NATO, told reporters at the Pentagon Tuesday that it is “likely” Russia is behind the attack, and acknowledged the alliance is “struggling” to figure out how to respond given the nature of the situation.

“As we determine the responsibility here, and it’s likely that it’s Russia, I think it underscores what they’re willing to do. To attempt an assassination on the soil of a sovereign country, I think, it should be very clear to use what they’re willing to do in order to further whatever their objective was here,” Scaparrotti said. “And I’ll remind you there have been several assassinations in Ukraine, as well.”

“This is a government that is violating all the standard norms and international rules and laws, to bring violence onto other nation’s soil in order to reach their objectives,” he continued. “Amazing, frankly.”

On March 4, a former double-agent named Sergei Skripal and his daughter were found poisoned in their Salisbury, England home. The weapon used against them has been identified as Novichok, a chemical agent developed under the old USSR.

Asked whether this was the first use of the nerve agent in Europe, Scaparrotti said he didn’t know offhand. But he said the use raises questions about Russia’s adherence to chemical weapon treaties — and if this was not a government-ordered killing, about Russia’s chemical weapons security.

More broadly, Scaparrotti said the alliance is “struggling” with how to respond, because while the U.K. and its NATO allies believe Moscow is behind the attack, it is unclear how a large-scale military alliance like NATO should act given the relatively small scale of the actions.

In some ways, it’s the non-nuclear version of Russia’s “escalate to deescalate” theory, in which Russia would be able to use low-yield nuclear weapons and leave NATO only two options — to not respond, or to respond with a full-scale strategic assault that would launch a world nuclear war.

“This is a relatively new area we’re dealing with, and we just have to start thinking about this and coming to terms with it,” Scaparrotti said of the situation, saying it shares some characteristics with the question of how NATO would respond to a cyberattack. “It will never be black and white, either, I don’t believe.”

As a result, NATO has started wargaming out some scenarios in order to try and be prepared for the next unconventional scenario.

“I don’t think it will ever be finely defined,” Scaparrotti said. And ideally, that results in a situation where “we don’t have to have that larger discussion if something happens in a crisis, we don’t have to have the larger discussion in the crisis. We can bring what we learned to the table, and deal with the present crisis.”

Aaron Mehta was deputy editor and senior Pentagon correspondent for Defense News, covering policy, strategy and acquisition at the highest levels of the Defense Department and its international partners.

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