ADMIT NOTHING, deny everything, make counter-accusations: Those are the rules for spies when their cover is blown. And so the Russian regime of President Vladimir Putin, a former KGB officer, is responding with bluster and threats to the German government’s proof that Mr. Putin’s leading domestic opponent, Alexei Navalny, was poisoned with a nerve agent while on a domestic flight on Aug. 20. Mr. Putin’s foreign ministry announced Wednesday that Chancellor Angela Merkel’s demand for an investigation of what she has called “an attempted murder” amounts to a “hostile provocation against Russia fraught with consequences for the Russian-German relations and a serious exacerbation of the international situation.”

Fortunately, Mr. Navalny, recovering at a Berlin hospital, has awakened from a coma and appears to be doing better, though his long-term health is uncertain. The question remains, how will Ms. Merkel and her fellow leaders of democratic countries hold Mr. Putin accountable for this latest human rights outrage?

This would be an easier question if the United States were not led by President Trump, with his notoriously tepid attitude toward Mr. Putin’s repression of domestic dissent and other abuses. On Tuesday, the United States joined its fellow members of the Group of Seven — Japan, Canada, Britain, France, Italy and Germany — in “condemning, in the strongest possible terms, the confirmed poisoning of Alexei Navalny.” Such words are no substitute, however, for a clear and convincing statement from Mr. Trump, of the kind other national leaders, such as Ms. Merkel and British Prime Minister Boris Johnson, have issued.

Germany — the potential leader of any unified European response — faces its own conflict of interest in the form of the multibillion-dollar Nord Stream 2 pipeline, near completion and scheduled to start bringing Russian gas to Germany next year. The Merkel government first hinted at using the project as leverage to force Moscow to respond more transparently, then dialed that back a few days later. Berlin would be unlikely to scrap the pipeline altogether, in part because it’s too far along, and in part because that would be seen as a capitulation to Washington, which opposes Nord Stream 2 and has imposed sanctions on European companies involved in construction.

Europe can make use of the existing international law framework on chemical weapons, leading, at least in theory, to United Nations sanctions against confirmed violators of the global ban on deadly agents such as Novichok, the Russian-origin one that felled Mr. Navalny. Germany took the first step by reporting the findings of its investigation to the Organization for the Prohibition of Chemical Weapons, though the threat is mostly symbolic given Russia’s veto power at the U.N. Security Council. More promising would be the adoption in Europe of legislation similar to the Magnitsky Act in the United States, to allow economic sanctions targeted at individuals found to be involved in human rights abuses. First proposed by a Dutch foreign minister in 2018, the law has been going through “preparatory work” in the European Union’s 27 members since then. The attack on Mr. Navalny should shock them into faster action.

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