Promising unwavering support for Ukraine, the U.K. replaces its ex-soldier defense secretary with a savvy politician.

When Rishi Sunak replaced Liz Truss as British prime minister last fall, White House officials said they didn’t worry about his support for Ukraine because he left in place the respected soldier-turned-defense secretary Ben Wallace, who had orchestrated Britain’s unstinting military support of the Ukrainians.

Now Mr. Wallace has stepped down, and in his place Mr. Sunak has appointed Grant Shapps, a politically savvy Conservative Party operative and close personal ally of the prime…