Tony Blair, Former U.K. Leader, Is Suddenly Back in Favor

A decade and a half after Tony Blair left Downing Street, one issue still defines the former British prime minister in the eyes of many Britons: his disastrous decision to join the U.S.-led invasion of Iraq.

When Mr. Blair was given a knighthood by Queen Elizabeth II last year, more than a million people signed a petition demanding the honor be rescinded. And within his own Labour Party, he remained a complex figure, detested by those on the far left while grudgingly admired by some who noted…