Tobias Billstrom, Sweden’s foreign minister, has surprised everyone by suddenly resigning from his role and leaving politics entirely. Billstrom, who played a key role in Sweden’s historic move to join NATO, didn’t give any specific reason for his decision. His announcement left the country in shock, especially since he was only 50 years old and had been involved in politics for over two decades.

Billstrom first entered Swedish politics in 2002 when he was elected to parliament. He climb...