Britain and France have promised to muster a “coalition of the willing” to secure a peace agreement between Ukraine and Russia. Now comes the acid test for Europe: How many countries will step up, and does that even matter, given Russia’s rejection of such a coalition as part of any settlement?

Prime Minister Keir Starmer of Britain left those questions unanswered as he bade farewell to fellow leaders after a summit meeting in London on Sunday. He conceded that “not every nation will feel able to contribute,” though he expressed optimism that several would, and that this would send a signal to President Trump that Europe was ready to “do the heavy lifting.”

Drawing Mr. Trump back into the process is as important as the mission and scope of a European coalition, analysts say. For the moment, the United States appears determined to strike a deal with President Vladimir V. Putin of Russia over the heads of Europe and Ukraine, and without any security guarantees.

Mr. Starmer presented his coalition of the willing as one of multiple steps that included continued military aid for Ukraine to improve its position on the battlefield, a seat at the table for Kyiv in any peace negotiation and further help with its defensive capabilities after a settlement. That is where the coalition would come in.

In addition to Britain and France, northern European countries like Denmark and the Netherlands seem obvious candidates to take part. Both have been strong financial supporters of Ukraine’s war effort and are NATO members who contributed to other security campaigns, like that in Afghanistan. Germany is the second-largest contributor of military and other aid to Ukraine, after the United States.