But it was not clear how strongly allies would rally to her side, and experts said that behind Mrs. May’s tough talk lay relatively mild measures, aside from the headline-grabbing expulsion, which she described as the biggest one of Russian diplomats in more than 30 years. That, in turn, reflects Britain’s weakened position in the world, as well as Russia’s continuing success in sowing discord and division.

“I expected a stronger reaction,” said Jonathan Eyal, international director at the Royal United Services Institute, a London-based defense and security research institute. He said Mrs. May might have calculated that in response to tougher measures, Russia would “decapitate the British Embassy in Moscow” with its own expulsions.

There are a number of tougher measures that Britain could also take, like seizing any assets of questionable provenance belonging to Russians who have invested and settled in the country, changing laws that made it possible to hide the true ownership of assets, and calling on the international community to tighten economic restrictions on Russia.

[Read more about Britain’s options against Russia.]

Britain’s broadcast regulator has also hinted that it could revoke the license of RT, the Kremlin-controlled English-language news channel.

Britain wants the support of its allies in taking action against Russia, but relations with those allies are shakier than they have been in generations, given Britain’s pending divorce from the European Union and frictions with Mr. Trump.

The European alliance itself is being sorely tested, with the rise of far-right and anti-Europe movements in Italy and elsewhere, defiance from autocratic sounding governments in Poland and Hungary, and the long struggle in Germany to form a governing coalition.