A nightmare scenario has been playing on eastern European minds with increasing intensity since Donald Trump returned to the White House: what if Russia attacks and the US does not join the fight? On the rare occasions the question is posed out loud, nobody much likes the answer. In mid-May, at a gathering in Tallinn, the US undersecretary of state Thomas DiNanno was asked directly whether American troops would fight if Russia invaded the Baltic states. He shifted uncomfortably in his chair, then gave a meandering answer. It did not include the word “yes”. Politicians from the region usually try...