A summit of European leaders to discuss Ukraine's defense has been overshadowed by the diplomatic row at the White House.

KYIV—A combative White House meeting has presented Ukraine with the prospect of fighting Russia without the support of its primary military backer through the first three years of the war. Now, the question is whether Europe has the firepower—and the political will—to help Ukraine hold off the Russians.

European leaders rallied around Ukrainian President Volodymyr Zelensky after the fracas with President Donald Trump. But arming Kyiv alone would be a struggle.