This week, asked by ABC News if he considered Vladimir Putin a “killer,” President Biden responded in the affirmative. Putin responded by challenging Biden to a debate this weekend. At this point, Donald Trump’s allies immediately weighed in on Putin’s side. Putin “is openly mocking the president of the United States for his own amusement,” Fox News host Sean Hannity proclaimed last night, “and frankly, I find it, as an American citizen, humiliating.” Donald Trump Jr. taunted Biden for lacking the guts to take him up on it: Trump consistently declined to acknowledge Putin’s grossest human-rights violations. When other Western leaders denounced Putin for poisoning his primary dissident, Trump evaded the question and never even conceded Putin did it. But why didn’t Putin ever challenge Trump to a debate? The answer is that they had nothing to argue about.