For years, the US security establishment has been wishing for Russian President Vladimir Putin to get served a cup of tea with a touch of poison. But Washington should be careful what it wishes for. With Putin reportedly about to go under the knife for a cancer surgery, Putin’s right-hand henchman, Nikolai Patrushev, is poised to temporarily take over, and he could be even worse than his master.

Patrushev is the most influential person in the Kremlin bureaucracy and is the only person Putin trusts — to the extent he trusts anyone. The two share more than a strong professional relationship. They are friends. Both Putin and Patrushev come from the same stock of Chekists, the term that refers to Soviet and Russian intelligence services known by the Russian acronym ChK, the powerful secret police agency in the USSR. Chekists view themselves as elite warriors, dedicated to protecting the fatherland from its enemies.

Both men have a military background, are trained to use weapons and famed for their brutality. During Stalin’s era, known as “Red Terror,” Chekists eliminated “enemies of the people” using a “special tradecraft” — executions by a shot in the back of the head.

Putin and Patrushev’s friendship dates back to their KGB service, which Putin joined in 1975, one year after Patrushev was recruited. Both at different times also served as heads of the FSB, Russia’s domestic security service, a successor of the KBG and a rough equivalent to the FBI. In 1998, Patrushev took over from Putin as Deputy Chief of Presidential Administration under Boris Yeltsin.

As an Army general, Patrushev outranks Putin, who only reached the title of colonel. Nevertheless, Patrushev acts as a “devoted armor-bearer and like-minded accomplice” to his commander in chief, according to a prominent Russian tabloid, playing a highly influential role in the formulation of Russia’s foreign policy and security strategy. As the head of Russia’s Security Council since 2008, Patrushev has almost certainly influenced the country’s modern military doctrine, which formally codified NATO as Russia’s top security threat.

Patrushev also rivals Putin as a villain. Some believe the two ordered FSB officials to bomb apartment buildings in Moscow, killing between 200 and 300 residents in 1999, in order to blame Chechen terrorists and give Russia a pretext to unleash war on Chechnya. As a result, the popularity of then-Prime Minister Putin rose, helping him secure the Russian presidency in March 2000.

After Alexander Litvinenko accused Putin and Patrushev of orchestrating the bombings in his book “FSB Detonates Russia,” he was poisoned by radioactive chemical Polonium-210, an assassination that British Supreme Court Judge Robert Owen concluded was “probably approved by President Putin and his Chief of FSB, Patrushev.”

Both men have likely authorized the poisonings and killings of many Russian “enemies.” As Head of the FSB, Patrushev led a counter-terrorism program calling for the “liquidation of the leaders of the Chechen separatist movement.” Patrushev dispatched FSB operatives to hunt down and kill Chechen fighters, including Shamil Basayev in July 2006 and Aslan Maskhadov in March 2005.

The FSB hitmen who carried out these targeted killings were given awards. The hitmen who were less successful got killed themselves, with Putin and Patrushev laying flowers at their funerals.

Patrushev doesn’t drink hard liquor, instead preferring the occasional glass of white wine. He is never late and doesn’t like speaking with journalists, a professional habit that comes with being a spy for 50 years. But when he does, he carefully calibrates his words, highly aware that his speech is being analyzed by foreign intelligence services. Like Putin, Patrushev is a workaholic and always has an opinion, which he shares with his boss daily. While Putin is a judo master, Patrushev is a fan of volleyball and swims one kilometer in a pool four days a week, to shake off fatigue.

Patrushev was likely one of the architects of Russia’s invasion of Crimea in 2014. He also helped create a series of top-secret military plans, including — it is believed — the broad operational concepts for Russia’s invasion of Ukraine.

Like Putin, Patrushev is a rabid anti-American propagandist, who recently accused the United States of planning to drop a nuke in Ukraine and then blame Russia for it. On Wednesday, he also blamed the West for seeking to destroy Russia.

There’s an old Russian saying. Literally translated, it means: “Two boots make a pair.” And Putin and Patrushev fit together perfectly.

Rebekah Koffler is the president of Doctrine & Strategy Consulting, a former DIA intelligence officer, and the author of  “Putin’s Playbook: Russia’s Secret Plan to Defeat America.” She also wrote the foreword for “Zelensky: The Unlikely Ukrainian Hero Who Defied Putin and United the World.”