Robert F. Kennedy Jr. sipped coffee inside the historic New Hampshire State House as he chit-chats with a gaggle of the state’s political leaders about the travails of being dad to seven children. Kennedy described his shock upon finding credit card receipts that indicated Conor—his 21-year old son—had recently embarked on a secret military mission. Without a word, Conor (who once dated Taylor Swift) skipped his law firm internship and volunteered to join the International Legion of foreign fighters on the frontlines, alongside Ukrainian special forces. With flashing blue eyes, gleaming...