U.K. Takes a Hawkish Lead on Russia, but Will Europe Follow?

The risk for Britain, Mr. Powell said, is that it is viewed in other European capitals as merely a “running dog for the Americans.” That is convenient for the Biden administration, he said, because “they can always rely on him to come out and say something even more extreme than they do.”

Mr. Johnson has forged a highly visible relationship with President Volodymyr Zelensky of Ukraine, calling him virtually every day. On Mr. Zelensky’s virtual speaking tour of foreign parliaments, his first stop was Britain’s House of Commons, where he singled out Mr. Johnson to thank him for his support.

“Having someone who is out on a limb is helpful because you can use them to shame other countries,” Mr. Powell said.

For Mr. Johnson, the war in Ukraine has utterly eclipsed a skein of political scandals at home. He has certainly shown an astute capacity to seize the moment. Last week, he recorded a video for the Russian people in which he tried to separate them from Mr. Putin. “Your president stands accused of committing war crimes,” he said in passable Russian. “But I cannot believe he’s acting in your name.”

On Friday, Britain imposed sanctions on two daughters of Mr. Putin and a daughter of the Russian foreign minister, Sergey V. Lavrov. They were the latest in what critics say is a long-overdue effort to crack down on wealthy Russians, many of whom have poured ill-gotten gains into London real estate.

“Before the crisis, the U.K. position was seen as hypocritical because of the failure to tackle Russian wealth flowing into London,” said Malcolm Chalmers, the deputy director general of the Royal United Services Institute, a think tank. “But now it’s done things that people have been urging them to do for years.”

Mr. Johnson has also pounded away at the need for Europe to reduce its dependence on Russian energy, a goal that is easier for Britain since it is far less reliant on it than its Continental neighbors. On Wednesday, Britain announced it would stop buying Russian coal and oil by the end of 2022, and gas “as soon as possible thereafter.”