Daniel Swift’s nerves were shot. By the start of 2019, his Navy SEAL colleagues said, he was hardly eating or sleeping. He had separated from his wife. A court had barred him from seeing his four children, and he was facing legal charges for false imprisonment and domestic battery. Mr. Swift told fellow SEALs in San Diego, where he was based, that he was planning to go to Africa to fight wildlife poachers. They brushed off the comment, convinced that Mr. Swift, a soldier’s soldier, would never abandon his post. A week later, he disappeared. Navy investigators searched for him,...