THE DEADLY nerve agents known as “Novichok, ” or “the new guy,” were developed in the final years of the Cold War. They are binary weapons in which two compounds become toxic when combined. Now it is imperative to get a detailed accounting of how this killer substance made its way to Salisbury, England, this month to poison a former Russian spy, Sergei Skripal, and his daughter, Yulia.

Britain, France, Germany and the United States have pointed at Russia and President Vladimir Putin, saying Moscow either deployed the nerve agent or lost control of it. As a next step, a comprehensive forensic investigation is essential for all nations grappling with how to respond to Mr. Putin’s aggression. In an earlier killing, that of Alexander Litvinenko, who was assassinated in 2006 in London with radioactive polonium, an investigation traced the path of the material on airplanes and in hotel rooms, showing that it probably came from Russia. A similar trail must be uncovered now.

Fortunately, there is an organization up to the task. The Organization for the Prohibition of Chemical Weapons (OPCW) at The Hague was established by the Chemical Weapons Convention of 1993 to rid the world of the scourge of chemical warfare. A lot has been accomplished — the treaty now has 192 state parties, and more than 96 percent of the declared stockpiles have been eliminated — but new threats keep springing up. Investigators from OPCW identified undeclared chemical-weapons activity by the Syrian regime and also usefully probed the 2017 attack on Khan Sheikhoun, an opposition-held village where nerve agents killed at least 87 people. This sort of gumshoe dedication and know-how is needed again.

Russia has also been implicated in cyberattacks on U.S. and European nuclear, water and electric facilities. Such shadowy maneuvers, often hard to attribute and slow to be discovered, are just the latest in Mr. Putin’s toolbox of asymmetric tactics against the West, which also includes fomenting political chaos, splitting alliances and hyping new military threats. The sanctions imposed so far on Russia are the beginning of a response but cannot be the end. The United States and its allies need a serious, long-term strategy for dealing with Russia that is clear-eyed about Mr. Putin’s capabilities and intentions and recognizes his highly personalized system of authoritarianism. If he insists on a new Cold War, the West must respond. While certain threats can be reduced with arms-control agreements, others may require building up defenses, such as against cyberattacks, or finding new means of deterrence. The United States must also find a robust voice to speak up for the truth when Mr. Putin hides it, as it did in the last Cold War. So far, unfortunately, President Trump has not found that voice.

Mr. Putin uses feints, leverage and camouflage. The West must peel away the layers, be patient and resilient, and not fail to expose an assassination attempt for what it is.

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